I'm a bridesmaid for a wedding this July. 4/6 of the wedding party are from out of town. How we're handling it is that we set aside a weekend about 6 weeks before the wedding to hold the bachelorette party. I cleared it with all the bridesmaids, and they are all planning to be there. It's going to be in the bride's hometown. Obviously not everyone we invite will be able to make it, but for us, the main factors were 1. convenience for the bride and 2. trying to pick a time that worked for the bridal party. After that, it's really up to the guests if they want to come (or are able to). So with that in mind, obviously my personal preference would be to pick a time before the wedding, and plan a party in Home Base location.
You really only need to know who's invited to the wedding, not who's coming, and go from there. I wouldn't put so much thought into whether or not they are likely to want to come or travel or whatever, I'd just invite them and let them decide for themselves.
I also wouldn't plan a party in the day or two before the wedding. First of all I think there's enough going on at that time so why add to it, and as others have mentioned there's usually a rehearsal, and dinner, and time with out of town relatives, etc. Getting people to fly in to the wedding city 2 days early is a big ask, not only are you asking them to take extra vacation time, but it also means paying for more nights at a hotel, more meals out, more transportation/entertainment costs (because they probably won't want to sit around doing nothing in a hotel for 2 days). I know there's similar costs involved for a party prior to the wedding, but if it's over a weekend there's less vacation time, and chances are higher that they will get to spend more time with the groom and other friends (as opposed to the frantic busyness in the 2 days leading up to a wedding.)
Oh, and for a bachelor party I'd go with a Facebook event or an E-vite rather than paper invitations. Plus having a facebook event page is helpful for posting updates, costs, other activities that might be thrown together for out of town guests, etc.
Edited for clarity