Heres why

Agree, Plat at SPG is a must, otherwise you might as well be a first time walk in without any status.. for some reason, Gold is virtually useless at SPG.. if you are going to do more than one stay at SPG, the plat is nice.. upgrades, club lounge access, and of course the 500 point plat amenity which IMHO is huge.. since you only get 3 Pts per $ at SPG.. a 200 buck night room only yields 600 spg points but when you can add a 500 pt plat amenity (250 at 4 points) its a significant amenity.. unlike the same amenity at Marriott where 500 points is worthless

Marriott, has the 2nd best mid level program, some will argue Hh gold is and I can see why they say that too.. Marriott is not huge on suite upgrades but they do have plenty of lounges.. club access is the key here and the difference in elite bonus is 5% or almost nill.. If it were my choice I'd make MR my mid level program

Hyatt, Diamond is amazing, you want this.. now I know you don't stay there but I think if you did and as a diamond, you would probably stay more.. why.. upgrades are decent, RC access is always nicer than the other properties in other chains, they treat you better than any other top tier level at the other chains and the only down fall is Hyatts are usually pricier and there are fewer of them... however with the advent of Hyatt Place and the Summerfield suites coming into the picture, they have somewhat alleviated the problem.. factor in the best promo's in the business in FFN's and I think you will become a believer

Hilton.. this is my midlevel.. I was diamond for eight years, a slow down in biz travel has made me choose one chain as its mid level.. I chose hilton why.. Hilton Gold is pretty good.. the top tier Diamond has one distinct advantage, the diamond line.. they can usually do wonders for you.. after that though, Gold gets you almost as much as anything Diamond does... What made me select Hilton vs Marriott as a midlevel is that 1.) its much easier to obtain, but 2) the real one is that I don't find many upgrades at hilton properties.

for the record, I'm Plat at Marriott (just the most selection so to me it pays to be top tier there); Diamond at Hyatt (best service hands down, love the promos too); Plat at SPG (I'm not sure why I keep this other than I manage 25 stays a year there and Gold is worthless in this chain). Hilton (after 8 years of diamond, I just didn't see the advantage of keeping it) Mind you the Hilton program has the rolling tier so you only have to qualify every other year if you time it right. So in a worst case I can make a run for Hilton in a short period and get status for two years. It just makes the most sense for me to keep Hilton as my mid tier.

hope this helps