Generally speaking the longer the shank the more leverage you are going to get out of the bit. So a longer shanked bit is going to be stronger than a bit with short shanks.
5 1/2 inches isn't terrible, but I would only use it on a well broke horse.
The aluminum is really just to make the bit look fancier, but it's a lot cheaper and lighter than silver. I've never owned one so I don't know how well they hold up. I actually like the first bit you posted better. Those copper rollers will give the horse something to play with and encourage salivation, plus I think it looks nicer for showing. Actually it's pretty similar to the one my friend uses on his pleasure horse. I don't think it will be a huge leap to go from one to the other though. The aluminum bit doesn't have as much curve to the shanks, but I think you'll be ok.
I hope that helps. If anyone else wants to jump in here feel free!