Here are my thoughts on these two.

First, the platform and method of using them are pretty comparable, as you've noticed. Both will be a big improvement over your MTB pedals in that respect.

The float is different. Shimano cleats are either yellow (6 or 6.5* of float) or red (0). Keo cleats are red (9*), grey (4.5*), or black (0). The red Keos were too much for me. I really like the grey. I was thinking of switching to Shimano, but I'm concerned 6.5 will feel too loose still.

The reason I have thought about switching: cleat wear. Keo cleats wear out much faster than Shimano cleats. The wear points on the Shimano cleats also don't really impact how the cleat sits in the pedal as much. Say you wear down the yellow toe point. It should still feel the same in the pedal. If I wear down the front of a Keo cleat from stopping and starting, eventually my knee starts to hurt and I need to replace them, even if they are not too thin to really risk breakage etc. As Keo cleats wear, they get harder to clip in. It helps to file off the rough edges a bit. The new grip cleats do wear a bit better b/c you wear down extra rubber bits. However, they require a bit more accuracy and force to clip in over the older style cleats. None of my friends with SPD-SL cleats complain about them at all. Everyone with Look pedals complain about cleat wear. The good news is that the cleats are cheap. I've also broken a brand new Keo cleat and had friends break their cleats at inopportune moments, which can be pretty bad.

So.. that's something you might want to consider as well.