Jon, it is hard to compare tm's because they are designed differently from the start. some machines are choked down for more heat, some are run slower to last longer, etc.
Prochem used the 47 blower on its 405 machine to reduce noise while at the same time being a marketing angle. If you want a good single wand machine a 2545 would fit the bill nicely. here are some things to think about:
an air-cooled engine needs to run at high rpm's to cool itself. get a oil cooler and use synthetic to help the engine. A 45 blower is 2.5 inches in and out so it works great with 2.5 inch hose and if running the blower at 85 % of max. rpm's will probably do a 300 foot hose run very well. if you have a 150 foot hose run you can always run two lengths of 2 inch hose to help bring the vacuum closer to the wand. A well made vac relief spring is important to hold the higher lift needed for long hose runs. Are you concerned with blower noise? if so talk to Les Jones about the blower silencer he uses. what is your need for hot water, and do you like high flow? the smaller machine will usually provide less hot water than a bigger machine. but you can always add a propane or diesel heater to make up for that.
Make sure to talk to the TM builders to get a sense of why they designed the machine the way they did. Duane Oaxley seems to specialize in building machines with 36 and 45 blowers. He is very easy to talk to. I think won of the most over looked aspects of cleaning is which wand is being used. that wand is the only connection between the carpet and the tm. A cheap wand can close down under high lift and not extract as well as it would at lower lift levels. there is a reason why lots of cleaners here use a Prochem wand and not a less expensive one made in china.
Hope this helps you alittle. a TM is really a system that has parts designed to work together and looking at just one part doesnt always give you the whole story.