The main problem sluggas have is that they are close combat troops. The reason this is a problem is that they have a low initiative so they nearly always lose guys when they get into CC. If you come up against another heavy cc army and only have sluggas, you may have a problem.
When judging a unit I always try to think of it performing its job in my army, not just in isolation. First and foremost, the job of ork infantry is killing other infantry. They probably have a power claw so they can take tanks apart if they get close, but that isn't their main role. As scoring units, they also have a key role in winning you the game by grabbing objectives. So which is best?
As I see it, enemy units you need to kill are going to come in one of three types: Stuff that you can't beat in cc with either sluggas or shootas, stuff you can beat with either, and stuff you can beat with sluggas thanks to the extra attack, but not with shootas.
Sluggas are only helpful in the third case but shootas are always useful unless you are shooting at something too tough for strength 4 to damage - in which case the sluggas aren't going to do much good either. In fact, it's difficult to think of a unit that a charging unit of sluggas would kill but shootas wouldn't. After all, the shootas are going to fire on the way in, so you end up with about the same number of attacks during the turn. You trade hitting on a 5 with shooting for the fact that you fire before the enemy strikes back.
Where sluggas are significantly better is in continuing combats or if they get charged themselves. Then they have 50% more hitting power. On the charge they only have 33% more.
The fundamental problem then for the sluggas, as cc troops, is that they have to charge even if they don't want to, as othehrwise they are doing nothing. It's very difficult to use sluggas to defend an objective, because anyone who comes to take it from them probably won't get shot at all on the way in. Shootas give you more options and flexibility, in exchange for a bit of straightforward hitting power in cc.