I think this is a case of "If you have to ask, you can't afford it"

The "gold standard" is Keil, there are others that use a "common base" and a " '51 adaption" these tend to produce much less efficient code. Then there is Raisonance which try to be a Keil clone, but if you visit the Keil forum you will se that "compiler error" (almost) always end up with "oh, I see, my bad" whereas in the Raisonance forum you see "will be fixed in next release"

I evaluated the bunch (not using eval versions) for a company a good while ago (my findings can be totally invalid today) and the first runner up was HiTech (now HTsoft), but they have concentrated on the PIC for so long that who knows where they are in the '51 at this time.

My opinion: buy Keil, if you can not afford it go SDCC.

Erik