If you're talking about Monster Lair, it's one of the most fun games on the system.
I find Shubibinman 3 relatively easy, unpolished and way too short. There's only something like 4 stages.
I'm guessing that you have yet to get the hang of either game.
I've only played each one once - Shubibinman isn't too bad, I got to the snow stage. The stages seem quite long.
But Wonderboy... I can't really see how it could get any better. It's a pretty basic concept, and I can think of many games in my collection that are more fun (IMO) let alone the whole system...
But I will say, Shubibinman (as a series) is clearly an attempt to better Mega Man, and the ways they thought they'd bettered it turn out to be its downfall.
*edit* £1.75 I paid for Wonderboy. It's not bad for that price. I've been getting some crazy bargains recently.
Wow, check this out, my most recent eBay PCE buys:
Conan future boy £2.20
Emerald Dragon £2.20
Op Wolf (loose) £4.20
Space Harrier and R-type £2.70
Energy £2.11
Wonderboy III £1.75
Shadow of the b £0.99
Ys I and II (TG) £3.20
Red Alert £4.97