You are correct, Miami is the "best" location, from a standpoint of latency, but latency is just a small portion of the factors that must be considered.
Finding a host that explicitly supports WordPress, as a reseller, will probably not be something you find. Managing WordPress requires a lot of human time (not server time), and is thus costlier to maintain. WordPress hosts want to deal directly with clients, not middlemen. If you want to be a reseller, you'll need to supply the WordPress management on your own.
For Miami,
Site5 is easily the best choice.
But for Costa Rica, Phoenix and Dallas locations should be pretty decent as well.
Note:
Hostdime is in Orlando, but they don't have the best reviews either. They've been downgraded in our reviews twice now, and are about to have it happen a third time. They may be dropped altogether this month.
WebhostingBuzz is an excellent host. We use them for a port of this site, both their US and UK locations.
Veerotech is an excellent host, and we use them for our major secondary projects, as well as a portion of this site.
Namecheap has excellent uptime and servers, but the support is better from the WebhostignBuzz brand. They're not the same company, but related (like siblings). Support quality is the same, but
Namecheap averages 2-3 hour responses, while WHB is about 1 hour (sometimes faster). NC also powers a portion of this site, as well as our major secondary projects.
Note: If you use any of these hosts, remember to use our links. While our advice is NOT influenced by monetary kickbacks, the modest funds received are one way we support this site. (If it was about money, then we'd be suggesting EIG brands. But it's not, and we don't!)
If you have any more questions, ask.
Each of the hosts you're looking at is one that we use ourselves, and have for years now.
The only thing you did not mention was budget? What are you looking to spend?
And what are you charging your customers? What I often find is people want to pay the minimum, and resell for the maximum. That never works out. For example, if you have 8 clients, and charge them $25 each per month, then you have $200 income to play with -- and surely you'll land more clients. So $50-100 is not unreasonable for the hosting.