I just started playing the bass in July, and I think I've gotten a pretty good feel for it. I use a squire jazz bass for now, and am planning to upgrade probably by the end of this year. My first question is.. how big of a difference will I notice if I upgrade to, say, a Fender jazz bass (possibly a geddy lee custom one). On that note, what are the main differences between jazz basses and precision basses? The guy at the music store wasn't too clear on that.

Next question... I'm using a Behringer 15 watt bass amp, but I have a feeling it isn't too good. It's not very clear and I'm having trouble finding the right tone (although that may also be my bass). Can anyone recommend a better amp to upgrade to that is reasonably priced (preferably lower than $150)?

One last question... I started out with the fingerpicking style, since using a pick doesn't seem natural. However, sometimes when I'm playing a bassline, I don't always alternate between my index and middle finger (for example, sometimes I'll strike with my index finger 2 times in a row before striking with my middle). I think this is due to me focusing on my left hand fingering. Is this a bad habit, should I try to alternate between my index and middle fingers perfectly?

Thanks in advance for the help!
first thing, get a GOOD AMP NOT A GOOD BASS! A Geddy sig will sound like **** through a ****ty 30W practice amp! A 4000$ custom Alembic will sound like **** through a ****ty practice amp.

But your Squire Jazz will sound GODLY through a good Crate 100W 115 amp!

Think about it like this, your bass supplies the fundamental sound, and your amp filters this sound. If you have a ****ty filter, the sound will be ****ty, no exeptions!
Jazz basses have usually the J/J pickups with a smaller neck and close strings, P basses have the P pick up and a wider neck. It's all preference, but as for Pick up tone difference in the J's and the P's....i'm trying to figure that out myself.