do you think a brand new 48x burner which also reads at 48x isn't as good as the "pretty old" Plextor 40TS ?

i've got both drives so which one should i use ?

How about testing which drive gives the best results on your machine...?
I think both drives should work fine. The main difference should be the ripping
speed of the drives. But again nothing will tell you better than your own test results...

There is only one way to find out: Rip 200 random cds and listen to them. If you did not encounter a single error, I'd say the drive is okay. Otherwise, I'd stick to the drive I trust. There is little more annoying than listening to a piece of music with errors in the ripping. I have tried that twice on 400 cds.

First time I ripped 400 cds using three different Plextor drives: 40TSi, 32TSi and a 8X burner. I had many errors. The Plextor 40TSi broke soon after - been using it way too much over the last 3 years. I assumed it caused the errors.

I was unfortunately wrong. Second time I ripped 400 cds with the 32TSi alone. I encountered many errors again.

My new (1 year old) Plextor 40TSi has however ripped hundreds of cds without a single error.