oem or after market?

I would say go oem, but suzuki oem parts are way more than the other brands. You may be able to get good after market stuff and save money also. Ask around where you live, take a look at thumpertalk,and maybe someone here will answer that has experience. I run a rm250 2 stroke and always go oem, but a 4 is different.

no experience with suzukis but going oem 99.99% of the time it will fit perfectly and everything is made for that model. so you can not go wrong with oem

The 05 RMZ is actually a Kawi with yellow plastic. If you buy oem parts the kawi prices are usually a little less than suzuki prices for the same part.

i've always used wiseco and had good results. Their kits are usually cheaper then piecing together an oem kit.

Thats not as true anymore as it used to be. Wiseco prices keep going up. I looked up prices from Kawasaki for the bike in question for piston, rings, wristpin, and clips was $156. Wiseco piston kit (NOT complete top end kit with gaskets) is $227 from Tucker Rocky.
Edit: I just checked Suzuki prices for the same parts are actually $4 cheaper in this case at $152

look at the JE pistons, they are great and not that much more expensive, but i have always had good luck with wiseco also