I have an Evenflo Triumph, and I'd give it a mixed review. The harness straps adjust tension at the shoulder, so when the seat's in the rear-facing position and the belts are BELOW the baby's shoulders, they can be hard to tighten because the baby's body is pressing against the straps. That makes it hard for them to move. Now that my child is facing forward and the belts are at or ABOVE his shoulders, I've had no problem adjusting the straps.
Secondly, the seat is rather wide. It's fine for my car, but it may not fit as well in your compact car. Personally, I think the seat's more comfortable that way, though. I also had looked at the Britax Roundabout, and it seemed like an older child would find it a bit narrow.
Thirdly, you have to unscrew the back cover to be able to remove the seat cushion. That's a big pain if you ever need to wash it.
Some people found the tension control knobs difficult to access while in the rear-facing position, but I didn't have any problem with them.
On the positive side, it received the highest crash test rating in the latest Consumer Reports review on convertible car seats. The Britax Roundabout had a crash test rating just below the Triumph, though it was higher in a couple of different areas. It's also a lot cheaper, but I don't think you're getting cheaper quality. I've had no problem moving it from one car to another. It's also very easy to move the harness straps on the Triumph because you can do so without rethreading the belts (or removing it from the car).
I think the only difference between the LX and DLX is the amount of extra seat padding and accessories. The essential seat is the same.
Good luck! I debated the same two car seats (I didn't look at the Marathon).