I would highly suggest a sump in whatever tank you start.
Pros
-The first and most obvious reason is that it adds more gallons to your tank, and as almost all aquarists belive, the bigger your tank the better your tank will do.
-The second main reason, and agreeing with fishman0729 on this one, it gives you some space to work with. I take my test water from my sump and add whatever chemicals to the sump also.
-Thirdly, the sump gives you an area to hide whatever you don't want to be seen , i.e. heaters, pumps, etc.
Cons
-The number one con is the problem if you have a power outage, if you do you better hope that your level of water won'r exceed your sump.
-The other con is that a sump cost money, I would recomend going to your local store and comparing sumps there and then create your own with an old 10 or 20 gal tank you have. It may cost you about $25 dollars to get the overflow, but I say its worth it.
Good luck all, message me if you have any other questions...