How does the scoliosis affect her car seat sitting? Is she able to sit well in a forward-facing seat? What about a rear-facing seat? Which way is she more comfortable? (I'd guess it might be rear-facing?)
Rear-facing is safer for EVERYONE as long as they are within height and weight limits of a seat that can rear-face. But, there are some conditions where comfort or convenience may outweigh the safety factors, and some parents decide at a certain age that forward-facing in a harness-- which is also very safe-- is "safe enough." That's a choice that is up to you, and I'd say at her age, comfort and convenience should weigh pretty heavily in the decision. You did great to rear-face her so long, though!
If you want to forward-face her, I agree, a combination seat (Nautilus or Frontier) might be a better choice for you. Babies R Us carries several of them in many of their stores, and lets you try before you buy.
If you want to rear-face, you should probably try different seats to see how she fits and whether she is comfortable in them. The My Ride has a higher weight limit, but shell is not significantly taller than the Marathon (if she still fits rear-facing in the Marathon with room to spare this might be a good choice for rear-facing her longer.) If she is closer to outgrowing it rear-facing by height than weight, the First Years True Fit or Sunshine Kids Radian seats might be better choices.
Since it sounds like her comfort in the seat will be a very important factor, if you don't buy locally, you might consider an online store with shipping insurance-- for $5, if the seat doesn't work, you're guaranteed to be able to send it back at no more cost to you.