Eh, they're both okay. Garfield's got far more merch and is much better known, plus his cartoons were much better written and more successful (even with Mel Blanc voicing Heathcliff in his last major role), but Heathcliff hasn't been overexposed to the same degree and I'd argue his comic isn't totally running on fumes the way Garfield's seems to.

There's room for both.