4 points to bring up
1: in D&D 3rd edition a rapier is considered a medium sized weapon that could be used with weapon finesse to give you the attack bonus with the dex instead of str. It is still considered a medium weapon though and using two of them WILL incur a -4 penalty to both hands, also if you pick up weapon finesse you can only hold one weapon. In the slightly different NWN the rapier can be used with weapon finesse, but you can use 2 weapons if you choose weapon finesse. Still, using 2 rapiers does incur a -4 penalty on your character.
2. Go to
Bioware, click on HoTU and in features take a look @ their epic weapon master. It's fairly good so you might want to take a look at it for your decisions. Basically if you want to do a class focus on a few main goal in that class... i.e. I want to do crit hits as much as possible, and hit lots of enemies at once etc. and select skills and feats according to your goal, thats it. Do whatever works for you.
3. A saber is traditionally a cavalry weapon, optimized for slashing attacks, and the rapier is a lighter weapon designed for thrusting attacks mainly. The triangle cross section is to give it strength so that it doesn't break in half when hit with a sword.
4. A rapier buffed up doesnt have a critical hit multiplier of x5... but you can get it to x3... Which is NOT as good but on the other hand you do a critical 55% of the time with the right feats... while a keen scythe doesn't get that bonus, so it basically balances out. Though personally I like the rapier doing lots of critical hits, especially combined with whirlwind attack...