Midlothian definitely has a more country feel than Rockwall. Both are good, dependent on what you're looking for. I think I prefer the surrounding areas of Rockwall more (meaning I prefer having decent access to some of the areas of north Dallas along Bush versus the areas along 20 in south Dallas). If you want a decent chunk of land, you're way more likely to find that in Midlothian than Rockwall. As mentioned, Rockwall, has a pretty good blend of small town feel, but it is growing and there are some parts of town where traffic can be frustrating - but just compared to the rest of town. It isn't like the kind of traffic you'll see in other parts of the metroplex.
As far as commute time, I can't speak too much to the exact location you'd be going to. I ride the train every day, so I drive about 15 minutes to Rowlett station, then spend about 40 minutes on the train to downtown. I don't like my commute taking as long as it does, but I can at least decompress, read, take care of other things while on the train.
I can say that on days when I know I'm going to be hanging around the office later, and drive in later, it takes about 30-35 minutes from my door to my desk. Last Monday, for instance, I left my house right before 9:00 and was able to be at my desk right round 9:30. Unfortunately, that doesn't work so nicely if you're talking about being at the office around 8:00 and leaving around 5:00.
I used to work in Greenville, and Rockwall was the furthest out I was willing to go at that time. At this point, all of our family friends and our kids friends are here, so we're not looking to leave, but the commute is definitely something you'll have to accept. It's all about trade off though - Dallas is freaking expensive if you're looking for sub 30 minute commutes (during rush hour) to areas downtown and want a little space. It's all about what is important to you.
