| Sorry folks...I just noticed how old this thread was! heh
I have recently bought rollerblades too, but they are aggressive skates. I will see if I can help some...
Lower numbers are soft, and higher numbers are harder wheels.
Basiclly, the softer the wheel, the more grip you will have while skating. So you may not maintain the speed as well as you might with a harder wheel. They will also wear down a lot faster then harder wheels, which would be a concern skating on some bike trails.
With harder wheels, you will be able to skate and glide a little faster and the wheels will not wear down nearly as fast as the soft wheels will.
With the Diameter, the larger the wheel, the longer and the more speed they will be able to carry.
Also, bearings play an important part in this. And also clean your bearings every once in a while to keep them spinning fast and freely, perhaps get some speed lube.
As far as brands go, both Fila and K2 make very very good skates. My girlfriend has a pair of Fila skates and she says they're the most comfy skates shes had, plus they haul ass. haha. It's hard for me to keep up with her.
I hope this has helped out some! Sorry I can't share more info being I have a pair of Razors aggressive skates with 54/88 wheels and a brand of bearings called Crap. haha  | 
