Ok so I have a situation where 1 pg is likely going to be a stud on offense. high spd, bh, pass. The problem is he is a serious liability on defense.

http://www.whatifsports.com/hd/PlayerProfile/Ratings.aspx?tid=0&pid=2301068

On the other hand, my other guy will not be as good of a passer or ball handler, by maybe 10-15 points in each category.

http://www.whatifsports.com/hd/PlayerProfile/Ratings.aspx?tid=0&pid=2301069

The first guy heitz has avg potential in all his remaining core categories for guard (spd, per, bh,pass, def)
The second guy saner is low per and ath, high in bh and lp and avg in the rest.

Is the defense a big enough deal to bench heitz for saner considering I will be losing some bh and passing?

Absolutely. Heitz's defense is atrocious, and starting him against the other team's (presumably) best PG would most likely lead to a very quick foul out. You may be able to "hide" him in the zone, but with 11 DE, it wouldn't be something I would do.  You could potentially start him if your opponent uses a PG that doesn't do much of anything.

I would bring Heitz off the bench, he could be a good option off the bench making it harder for other teams to game plan against him, which is what will happen if you start him.

That's what I was starting to believe. I figured I'd try him for the first couple games and see how it goes, but results have not been good.

I agree with the above. I'd bring Heitz off the bench and try to match him up against your opponent's weaker scoring option to try to avoid foul trouble.

Bring Heitz off the bench. Coaches usually don't account for bench players so having a stud off the bench is a plus. His defense is too low to go up against other starting guards.