It would be cheaper. Better? That's up to how you see it. Your sump needs to meet your needs, that's all. The bigger, the better. If you can fit a 40b under your stand and it'll do everything you need it to do then it's perfect for the job. If you need more.....
Regarding fish, add fish from least aggressive to most. If your dream fish is going to be the least aggressive, add it first, if the most aggressive, add last.
Many fish will coexist with their own kind in a large enough tank but will murder each other in a small tank. In a 55 it's better to only house one of these types of fish in such a small space, or you'll end up with one, anyway. Best to research each fish before purchase. Actually, it would be best to research all fish you want, make a fish list then stick to it, but that is not always possible.
Good luck and keep us updated with pics. We love pics.