it really doesn't matter. The difference as you pointed out is that one's got HDMI, the other has a larger hard drive. I'm gonna go ahead and venture a guess that your gf's mother will NEVER use up 320gb, and if anything the HDMI will be more useful if she wants to hook it up to a TV to show pictures of a vacation to family and friends or something.
Buying from a big box chain usually amounts to higher prices and tax, whereas buying it online you're not as subject to that stuff, but you've gotta pay for shipping and wait for it to get there, also you don't have a direct person to go yell at if something messes up
They're pretty much the same laptop, so it comes down to where you feel more comfortable buying from. Also keep this in mind if when you're getting it, if she gets it from office depot, and she has a problem with it, she's gonna go directly to them. If she gets it off TD then chances are she might go to you and say hey you recommended from them what do I do I've got "XXXX" problem. Working (and living) with adults who are more out of the loop than us kids are with tech, it's generally better for peace of mind to buy from a big box store so there's a living breathing person you can take it to, rather than get the run around on the phone and have to wait days or weeks just to get a new one, or run the risk of having it sent back to you with a note that says sorry you're SOL.
just my 2 cents, but ultimately it doesn't really matter because they're both the same laptop and they're a fine laptop for whatever she wants to do I'm sure.
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