A shower requires a 2" drain line and trap, so you probably won't like the step-up height. If you can use gravity, that is always the best choice rather than pumping it. You'll have to break the concrete to put a sump pit for the pump, regardless. So, why can't you tear up some more concrete to use a conventional height for your shower? If you can't break the concrete at all, you really can't put in a shower. All of the pumps/basins I've seen (there may be some) require it be below the height of the floor so the water can drain so you don't have the chance of the pump running dry.