If you don't think you're going to get that many miles, I'd pick UA. UA offers a transfer to Diners Club if you need to empty out orphan miles.
And since I live in neither a UA or US town, I just transferred 50,000 UA miles to 50,000 DC points to 2 Southwest Rapid Rewards tickets with DC's current promotion, and with the companion pass, converted it to 4 unrestricted round-trip tickets.