Southwest does not serve MEM, but they have good flight options for STL and MCI. If you fly 32 one-way trips they automatically check you in and you get very early boarding. That's it for elite perks, unless you can rack up 100 credits in a year with the help of credit cards and score a Companion Pass.

Since you're spending your own money, you might want to check out this and a lot more in the Southwest Wiki FAQ.

If you hate commuter jets and turboprops, Southwest might win on that score with its 100% 737 fleet.