I have the full size & K version MKE MP5s and a Colt 10.5" 9mm AR (had a 7" DOE upper for it for a while as well). I loved the MP5 ever since I first saw Die Hard and owning one is that much better.
MP5 Pros:
-A little milder recoil than the AR
-Suppression seems a bit quieter on the full size vs. 10.5" AR
-Factory HK mags are a little cheaper ($30-$50 each)
MP5 Cons:
-Parts/accessories are more expensive
-Gun is more expensive
AR-15 Pros:
-Manual of arms is same as 5.56 ARs
-Some commonality between parts (if you own other ARs)
-Compatibility with other AR accessories, stocks, grips
AR-15 Cons:
-Factory Colt magazines are expensive ($50-$75 last time I bought any, but Metalform & C-products are cheaper and work well)
-Recoil is more noticeable than the MP5
-Suppression is not as quiet as the MP5
-Direct blowback is harder on the lower receiver (make sure to get stainless steel pins and a ramped bolt)
In the end, if I could only have one, it would be the MP5, just because I like the look and feel of it. Where in Ohio are you? If you're around the Dayton area, you're welcome to join me at the range and try them for yourself.