In my opinion, the one with the bigger cache would be better.
The 40x CD-ROM drive will run very few CD's at that speed as they tend to become unstable and vibrate too much, infact even the 32x CD-ROM drive may have problems spinning CD's at full speed.
That is a theoretical maximum speed usualy only sustainable when using CD's with no printing on them as it is the weight of the ink which destabilises the CD's
The cache is substancially larger (4x) on the lower speed CD and you will notice a difference.
Can you get them demoed? Or perhaps link into the manufacturers home page and get all the average seek times and average transfer rates, both burst and sustained.
CDROMS, hmmmm, being a PC System Tech, and having used, Teac, Acer, Mitsumi, Asus etc, etc, etc. I would recommend getting a 32X, 36X or 40X. As long as it is from a major manufacture, and is the lowest price, then you have got the best deal. The differences are fairly minor. CDROMs are getting to be like 3.5" FDDs, do they work? yes, was it the least expensive? yes. Then you got the best one!!!