The biggest benefit I found from Summit status was the ability to change flights with no penalty. I don't think United gives that, even at the top tiers. Think I might take a trip in a couple months? Book a ticket... can always change it to something else later if I don't want to go. That was an amazing benefit for how few miles it took to reach Summit. I've since fallen down to Ascent due to lack of business travel, but plan to keep Frontier over United status.
I echo the previous sentiments when I say that I've never flown Southwest, and I never will.