The Viking or the Turf Tracer
For you I would recommend the viking. The turf tracer will have the floating deck and the viking will have a fixed deck. The turf tracer (as i'm sure you know) will cost more and be a more desirable machine but the viking would serve you just fine. I wouldn't worry about cost of operation on a 36" walk behind. It won't break the bank.
I bought my first commercial grade walk behind when I was 14 years old. I was mowing 6? yards a week maybe and saved up some cash. 48" Bobcat walk behind. Soon it was 25 yards a week and that thing kept up with it all just fine. I would break one push mower a season before that. Cost of maintenance on the bigger machine saved me money instead of fixing/replacing those push mowers all the time. Sorry to get all nostalgic on you- I've just been there, done that