You may want to surf over to Scuba Diving Magazine's website and look up their Scuba Lab reports. SDM rates fins every two or three years and gives lots of good tips on what to look for.
Splits versus blades often sparks heated discussions on this board, in case you don't know. People tend to swear by whatever equipment they use and for some reason many take umbrage when others swear by something else.
I had a pair of each until my son got certified and "inherited" some of my gear. Now I'm using Atomic Smoke On The Water split fins (great fins, clumsy name) exclusively. What I like about them the most is the spring straps, which isn't necessarily a function of the fins themselves. I'll probably pick up another pair of blades sooner or later, probably later frankly, but spring straps will be mandatory.
The biggest difference between splits and blades as far as I'm concerned is the kicking style, which is relatively easy to adapt.
As far as sizes, I recommend that you try them on before you buy and be sure to take your boots with you.
And Blackwood's advice about leg exercises is spot on.