First, thanks so much for all the replies.

My goal would be to either work on getting lots of domestic flights (seems to me that a Southwest card would be good for this, despite the imminent point devaluation, right?) or work on obtaining flights to Europe (seems like United would be best for this, yes?). With this in mind, I'm rather arbitrarily leaning toward the United Mileage Plus Explorer card. It's currently got the 50k bonus + $50 credit right now and first year fee waiver. The United also has no forex fee.

The only other one I really considered that was mentioned was Barclay's Arrival because the 2x bonus covers all purchases and thus accommodates my erratic spending habits. But, would the Barclay's Arrival be better with this goal in mind? What airlines could I use with these two cards? And will I run into issues transferring Arrival's points into a form that can be combined with my UR points? For whatever reason, I had trouble finding detailed information about this card.

Considering the credit history mentioned in the original post, what would happen if after a year, I simply cancelled the United card? Credit hit would be very small, right?

The problem with the United card is that it costs nearly $100 annually after that. Considering the fact that I really don't spend much, I don't think it would be wise to keep two annual fee cards going. My inclination would be to downgrade from Chase Sapphire Plus to the regular Sapphire in order to waive the fee. I really wish there was a way to downgrade the United instead, but I can't find a way to do so. In the end, it probably doesn't matter much anyway. I know that Chase gives 20% back when booking through the Ultimate Rewards' site, but I can't transfer the United points into my UR account. As a result, I'll have to transfer them out of UR to United and lose the bonus. Again, does this make Barclay's more lucrative?

A quick note: The Freedom card doesn't seem like a great fit for me, despite the lack of an annual fee. I don't have a car, so the gas benefit is useless. And since my number one priority is to keep all expenditures low, I feel that even the restaurant and Amazon bonuses (probably the most useful ones to me on the 2014 schedule) are only of mild benefit.