Could anyone help me resolve the differences between these two BMX bikes?
The heart of a bicycle is the frame - even on a BMX. Both of the models that you are looking at use high-tensile steel rather than steel alloyed with chromium and molybdenum ('cr-mo' steel).

With a normal bicycle made of high-tensile steel the problem is the weight - the frame (and fork) weighs significantly more than it would be if it was made of a better 'cr-mo' steel.

On a BMX bike the problem with high-tensile steel is not just the added weight. High tensile steel bends whereas 'cr-mo' steel flexes. You need this flex if the bike is to survive crashes as well as intended heavy impacts.

Given that there is a premium attached to 'cr-mo' frame/fork many manufacturers replace one or more tubes in a high-tensile steel frame with 'cr-mo' tube(s). They can then put a sticker on the frame claiming it is 'cr-mo'. This gives the impression that the frame is 'cr-mo' when it might be mostly high tensile steel. Therefore, when comparing bikes, look at the frame and fork to see how much of it is 'cr-mo' and how much of it is high-tensile steel. Get this detail right and go from there. Components can be replaced - if the bike comes with cheap plastic pedals then you can upgrade them to something more desirable when they break for a modest amount of money. So get the frame right and look for something with 'cr-mo' at least in the main tubes. 

EDIT:

The GT Article and the Mongoose Performer are very similar in specification. The separate seat clamp on the GT is a neat touch, the frame also appears to be chrome plated rather than painted. Personally I prefer the chrome plating as it can be polished up very easily to remove rust spots whereas the painted frame shows damage that bit more evidently. There is little to choose between the components, so on the basis of the chrome plated frame I would check that it is (and not just painted silver) and go from there.

The slight complication and opportunity is with the pricing. There are new 2012 versions out of both of these bikes. Some dealers may therefore want to discount the 'shop soiled' 2011 models that they have in stock so that they can get the 2012 models in. You may want to look at the differences in the model years to see what those differences are, maybe it only comes down to paint scheme, but, with a bit of shopping around, you might be able to get yourself a deal.