More details would help, but I'll give you my personal experience and reasoning.

I live within minutes of a US pseudo-hub (DCA), but fly UA. For me the reasons are:

• US just doesn't fly to several locations I need domestically. OKC is the most frequent and glaring example, but there are others.
• UA offers a much better international network. I fly domestically for business and internationally for fun and I want a "home" airline that will take me cool places.
• UA offers 6 SWUs at top-tier, US only offers 2. Combined with US not flying to Asia at all (where I tend to use my SWUs), they might as well offer none.
• It seems that US may be implementing something like *Net blocking now that they've sold off tons of miles. That always sucked with UA, but they seem to have moved away from it post merger. Domestic airline miles are best spent on partner premium cabins, so the harder a carrier makes it to do that the less valuable those miles are.