I have a pair of spd cleats that have thousands of miles on them as well as significant walking from commuting. The only downside is that there is some degree of play both up and down and forward/backward when clipped into the pedal. It is probably more frustrating to hear it than feel it since most shoes have some play in them anyway.

I also own look style cleats that have worn down to the point that I do not trust them much much more quickly without after market protection.

My vote would definitely be for the SPD style given that you are walking a lot.