Hi.
Having just gone through this decision, I can tell you a little about the side impact airbag differences:
98 has side airbags but no real head protection.
99 has modified side airbags to give more head protection (over the 98's)
2000 - I think that this is the year that they introduced front air curtain, which supplements the side airbag, but is specifically for head protection. The 2000 air curtain is for front passengers only.
2001 - Air curtain is extended to back seat. (still no side impact for rear passengers though).

At some point (I think in 2001) the front impact airbags became 3 stage airbags. that is, they will not go off at all if front passenger is below something like 30-40 lbs. They will only go off to something like 70% power if passenger is less than something like 110 lbs.

something like that.

not sure what is new for 2002 or 2003.