MTB pedals like this
are good for frequent stopping (commutes etc) as they are easy to get in and out of...and are super robust. Also, with an MTB or cross platform shoe, you can walk "normally" in the cleats...However without exception (for me) they have always developed an annoying creak on rides longer than about 30 miles or so ( I should be fair and add that I rode 1000 miles in them in one tour and they only creaksed a little...after frequent lubing). They also (again for me) can cause a numbness in the centre of my foot that spreads painfully to my little toes...after a long ride (80 miles plus). this may be due to shoe choice though as my shoes have a flexible sole
SPDSL pedals like this
Are less prone to causing numb feet as they seem to spread the contact area (plus tend to appeal to a rigid sole shoe) and seem to be creakless for a hundred miles or more.
My problem is I now have different shoes, cleats and pedals for all my bikes...its very confusing.