Maciek,

It's not a matter of fundamental mode crystals being "better" than overtone crystals. It is just a matter of applicability. The 8051 oscillator circuit is meant to make a crystal resonate at its fundamental frequency. If you want to make a crystal resonate at its third (or fifth) overtone, you *have* to incorporate some tuned circuit. In principal this is possible with any crystal, although devices meant for overtone oscillation might be cut slightly different.

Most manufacturers don't make crystals with fundamental frequencies higher than about 30 - 35 MHz, because they are much more difficult to make (the higher the frequency, the smaller/thinner the crystal).

Regards,
Rob.