Unfortunately most laptops (with exceptions such as a few Alienwares) cannot be upgraded beyond RAM and HDDs - neither of which would significantly improve gaming performance. I'd suggest looking into a desktop for the ability to upgrade.
I've had a similar situation, a pair of Skullcandy earbuds with an integrated mic, used with my laptop which had a combined mic/audio port, and the mic worked fine. You should be good to go.
Out of curiosity, why no splitters?
Personally I'd go with the Dell - Asus has a good reputation but based on specs the XPS should be a bit faster while offering better power efficiency (so better battery life).