5 boxing heroics to get gear is gauranteed gear. In multiple ways - you get emblems, you get blue and epic gear drops, you get rep that will allow you access to epic rep items, and they can be done very fast with no reliance on anyone else.

If you KNOW you have a very good Naxx group, there is a decent chance naxx will get you more gear, but that would depend on a lot of factors - you will be rolling against a lot of other people, if you're 5 boxing they might only let you roll once, if you're single boxing you'd have to run naxx 5 times which would take something like 30-40 hours in a pug.

Bottom line is you should do both. You can only run naxx once a week, but you can run heroics every day. especially when you first start out the gear from heroics will be great. I can honestly say I've never successfully pugged naxx - mostly because it takes >2hrs most of the time for the pug just to find a full group and get to the instance and start, and then if the pug falls apart due to a bad group you start over. Then factor in that you may not even win any drops and be stuck with only emblems.

The emblem/hr of naxx with a pug is much slower than the emblem/hr of heroics, especially if you factor in the heroic daily.

So a balance of both is good - but heroics are guaranteed gear, naxx isn't, and naxx would be far more frustrating on average. Also if you can't 5 box naxx, or you can 5 box it but can only roll once, then heroics will be much faster.