The Pro/Carbine and the Model 98 are two completely different styles of markers.
The Pro/Carbine is the last of a long line of markers and includes the best features from the Prolite and 68-Carbine, plus the main body is made from a solid lump of metal which makes it very strong. The Pro/Carbine is generally harder to upgrade and some of the upgrade have been discontinuted as the marker is due to be discontinued by Tippmann soon, i think they may have already stopped making them.
The Model 98 was the first of the new style clamshell designed markers so whilst being strong they are not quite as robust as the carbine range. They have a lot more upgrades available including a low pressure conversion kit and elecronic grip frame. This is currently a popular site gun, I think only the Inferno is more popular for fields in the UK.
If I was going to buy one of them now I would pick the Model 98, but I have a tricked out Pro/Carbine and actually prefer it to the Model 98, but if I was to buy a new recball/senario gun it would be the Tippmann A5 as that is the real successor to the Pro/Carbine. Oh and by the way the Pro/Carbine costs a bit more stock than a stock 98 due to the more expensive manufacturing process used involved.