My 2 cents...
With Trellis you can grab a spot to anchor (although arrive very early in the day as the anchorage fills up though maybe less so in July when you are going) and dinghy to shore. At Bomba's you'll probably be getting a spot in Sopher's Hole or Cane Garden Bay and taking a taxi - though there are plenty of taxis around the night of the FMP. If you come from CGB, make sure you lock your dinghy when you go ashore to grab a taxi. A taxi back to the boat (only if you are at the Sopher's marina) allows some of your party to stay later and others to crash earlier which is less of an option if all your charter members are piling into the dinghy for the ride back to the boat.
I went to a Bomba's FMP in 2012 as I was chartering with guys only. To be honest it didn't live up to the hype. It was pretty much all adults standing around waiting for something to happen and we saw less debauchery than say, Willie T's. But in fairness, I've only been to one FMP there. Despite being more 'family oriented', there is still a lot of partying that goes on at Trellis.
Trellis has more stuff going on with a couple bands playing and more bars with lines that you can grow old in while waiting for a drink (hint: for quicker service walk along the beach away from the action and get a refill at one of the tiny beach bars). The firepots are cool to see when in full blaze (I'd suggest you get there before sundown so you can see them get lit up). And it is easier to walk along the beach at Trellis whereas the waters behind Bomba's are heavy surf.
When all is said and done, it might come down to where you want to be at that point of your week and where you are heading the day after. Trellis would be more convenient if you're off to Anegada the next morning.
D
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