this is nothing but bad naming on nvidia's part. they are both the exact same performance wise i would guess but the newer one is just a 8800gts(g92) brought up to 9800gtx/+ specs. i have a older 9800gtx but the only diffrence there is the + versions are 55nm and mine is 65nm. what dose that mean to you.... short=nothing, long= it will run slightly cooler if anything. i think both use the same amount of power even tho the older one has two 6pin while the new has one. so honestly i doubt you would notice any performance changes from one to the other so i would say get the cheaper one and call it a day.
Found some benchmarks for 9800GTX vs 9800GTX+. This is a benchmark posted in a forum thread granted. But based on this guy's testing, there's little difference between the two. Albeit the 9800GTX+ does perform slightly better than the 9800GTX.
The + version is slightly faster due to it's higher speeds and can generally be over clocked to a higher speed than a non + version can. Unfortunately the move to 55nm provides almost no power savings at that high speed on that architecture .