The beach wedding - wear a pedicure and a smile.
Seriously, though - be prepared to carry your footwear, so make sure your feet look good. You can get those cute little bead things for your feet - a small loop slips over your middle toe and the larger bit goes round your heel. Nothing else to them, just jewellery for the top of your feet. I've made a few, with shells or beads and using that stretchy clear elastic yarn you can buy in spools.
The formal wedding - haven't been to one of those for a while, unless it's specified "white tie" I'd be maybe renting a suit, not a tuxedo. Or even check into costs and buy a suit - sometimes it's almost the same price (at least for us) and you've then got a suit in the wardrobe for a later (not so formal) occasion. Such as the divorce, maybe...?
"White tie" would mean tuxedos, tiaras and extreme formality; "black tie" means a two-piece suit is fine (no vest needed, but they're not expensive to add to the ensemble) with either a standard tie or bow-tie, black of course. For a touch of formality you don't wear one of those 'cheating' ties on an elastic band, you use a REAL tie (especially bow tie) because a real one looks so much more authentic, especially if it's not quite perfectly tied. A classic piece of snobbery within anyone's budget, because you can make it yourself really cheaply. Cheap to buy, also.
A formal wedding may not mean extreme formality of clothing; more possibly, it means a sit-down meal with the usual protocol of speeches in strict order according to all the wedding etiquette books; the photographer taking roll after roll of posed, unrealistic photos while the bride's cheeks go into spasm; a session of formal dancing (the bridal waltz, where the bridal couple take the floor first, followed by the bride's parents, then the groom's parents, then the wedding party, then the other guests (often bride's family first then groom's family). There is a formal cutting of the cake and other cute new formalities are creeping in. At a niece's wedding there was a table with two lit candles. The bride & groom each picked up their one white, lit candle and use them together to light one tall coloured candle, to signify their new life together. They then snuffed the two white candles to signify the end of their old, single lives.
A friend leaned over to the father of the bride and said, "What a lovely thing to do! Where did you get the idea for that?" perhaps thinking the custom was from the central European heritage.
"I got it from Survivor," he replied.
Another family wedding we went to, which the whole family now wants to forget, it was so awful - my brother's son was marrying the (regional) city socialite of the year. Her uncle the archbishop officiated; the reception was held at the seaside 'room with a view' (and it was such a grey day you couldn't see a thing, except the pounding surf eroding the beach - prophetic). My brother had said that husband & I were invited, but not the kids. He said none of the other kids (of our brothers & sisters) were coming either, except for maybe one or two. We got there, hotel room booked (too far away from where we live) to find that our kids were the only ones not invited. Hmm... We took our kids to the church bit (plenty of room there) and made sure they dressed up, then back at the hotel left them with easy child & BF1 babysitting in front of cable TV while we went downstairs to the reception. easy child by this stage was really miffed at being excluded, when all her other cousins were there (we'd by this stage seen them at the church).
The reception was formal in dress only. The usual speeches with careful order of who says what, to whom and why was simply not followed. When the guests were mingling, I could clearly see a gap in the room, between the groom's side of the family (ours) and the bride's.
I did what no sensible politician ever does - I crossed the floor. I tried to make conversation with the bride's family, only to receive a "What ARE you?" type of glare, then backs were turned again. I'd never met these people before and hopefully never will again.
Next morning we went down to breakfast, taking the kids with us. The room of the night before was now the breakfast room. Very little was left of the beach outside the window. The bride & groom were there at breakfast also; easy child & BF1 went over to congratulate them. Her cousin the groom then asked, "Where were you last night? I didn't see you at the reception...?"
"We were upstairs minding the younger ones. You see, we weren't invited," easy child politely but pointedly replied.
Formal wedding in regional city - cathedral wedding and reception in the BEST hotel. Cost - undoubtedly excessive.
Cost to them for inviting two of us - unknown, probably $50 total.
Cost of wedding present from us - $100
Cost of hotel accommodation for all six of us - $350
Cost of fuel to get there, plus extra meals - another $100.
But the look on the bride & groom's face when easy child told them why she had been missing from the crowd of supposedly not invited cousins - PRICELESS!
And my brother has not been allowed to forget the shabby trick he played on me & husband (a lot more to the story - he's had his knuckles well and truly rapped, and not by me, so I feel virtuous).
by the way, that marriage lasted five months. And knowing we never need to deal with members of that family again - also priceless.
With hindsight, we would have been happier not going to the wedding. It was really a very unhappy event in so many ways.
So check out tuxedo prices, think about how close you are to these people and make your decision. Sometimes it's fun to have an excuse to dress up especially if you enjoy being with these people. Other times you know it's going to be tedious and unpleasant and you don't want to have spent too much money on unnecessary frills into the bargain.
I vote for the beach - flowing muslin or light silk (but something that will take sitting on the sand) in a lovely floral or jewel colour, with perfectly manicured feet/hands and footwear made to be carried, not worn. Silk flowers in your hair - you can swim in them and they still look good. A handy towel, because at some stage, you WILL dip your toe in and you WILL get wetter than you intended. But you WILL have loads of fun, from the sound of it.
Then use the memory of the fun, informal wedding to get you through the formal one - if you go.