Either one of the approaches will work just fine...provided you know exactly what you're getting from each priming material.
The problem lies in the sensitive nature of bottle-conditioning. There's a relatively small target to hit when it comes to carbonation in glass bottles: there are only a few ounces of fermentable sugars mixed into five gallons (that's what's fueling the carbonation-building process) between under-carbonated, over-carbonated, and explosions ruining carpet, walls, and possibly injuring you.
The problem with malt extract is the fact that you don't know exactly how much fermentable sugar you're going to get out of a certain amount of extract. You can calculate for that and/or check some online resources and you'll be able to find out the amount of fermentables in the extract, but if you're wrong (or they're wrong) there might be some serious problems. The left-over yeast in the bottles are going to eat as much as they can, they don't care about blowing the bottle up and ruining your carpet (or face). Plus, you've got to worry about the freshness and flavor of the extract you're using to prime: are the flavors that are going in to the beer flavors that I want? What about later when you're trying to brew a beer with a very specific flavor profile in mind: how much flavor will that priming extract impart and will you be able to adjust for that?
Priming sugar is nice because it's almost completely fermentable (leaving behind very little of anything that will affect the flavor of the beer) so you have a really good idea of exactly how much fuel you're giving your yeast. Priming sugar (which is normally corn sugar, so it's readily available at the grocery store) is cheap, easy to use, transparent in the end product, and really efficient. I had considered using honey, different sugars, or DME to prime my bottles. I came to the decision that priming sugar was the easiest, cheapest, and safest.
Sorry for the long read, I'm always full of words and arguments.