Hi, Justin,
Your story sounds familiar, as I went through this myself a couple of years ago. Small format wasn't letting me do the large (4ft x 12ft) fine-art nature prints that I wanted to do. Like you, I never shoot in a studio, and wondered how these cameras would do in the field under available light.
Since then, I've been through a Phase One P45, P45+ and now have a Sinar eMotion75LV on a Hasselblad 500C/M, Mamiya AFD II, Hasselblad H2 and Sinar Hy6 respectively. And all this is after four+ years of research. (I was naive enough to think I could get it right on the first try...)
I'd say that there is little substitute for actually handling these cameras. Honestly, if I were to do it all over again, I'd book a flight down to Atlanta and visit the folks at Capture Integration and Professional Photo Resources to be able to play with and handle each of the camera over a period of at least a few days. You won't find better dealers, and you'll be doing yourself a huge favor in terms of familiarizing yourself with the handling and specific image quality tradeoffs of each system.
I can list pros and cons of each system as they stand for me, but that may have little relevance to you--let me know if you have any specific questions, and I'll be happy to answer.
Basically your MF solution (regardless of brand) is going to be bigger, heavier and slower. It'll have much more noise at higher ISO (with both Canon and Nikon with ISO 25,000 solutions the idea of ISO 400 being "high" seems almost comical these days, but 400's as high as I'll go with any of the MFDB's I've tested or owned for my fine art work, and even then only in a real pinch.) The workflow will not generally be quite as elegant (although there is a good degree of variation here in MF), and digital backs eat batteries quickly (300-500 shots), which becomes a consideration if you are on a trek for many days without power.
AF is in the 1990's as far as a small format shooter is concerned. Relatively slow, one center (or a few) AF points and no ultrasonic. In my opinion, any claimed differences in AF performance between any of the above MF systems is splitting hairs between systems which are all poor relative to the state of the art in small format.
MF lenses that have impressed me are the Zeiss FE line and the Rollei (Hy6/AFi) line (Zeiss and Schneider). Unfortunately the Hasselblad 203FE/205FCC has very limited digital back support.
I must admit a bias for 6x6 solutions as I do believe cost of sensors will continue to fall (remember the debates about whether APS-C would become the new full frame for 35mm??) The handling of the 645's just isn't as elegant (especially with a waist-level finder). If that's not important for you, then you'll have an embarrassment of riches to choose from.
My Sinar Hy6 body isn't quite as well built as a Hassy V body, but it is still quite nice. To my eye, the Hassy V viewfinder has a slight edge over the Hy6 finder, but beyond that the Hy6 pulls ahead in terms of performance (up to 3fps if the back can keep up), full electronic integration (no sync cables, which are a pain when changing lenses, or get caught on things out in the field), rotatable sensor solution (removing a back to rotate it is a non-starter for me in the field) and ergonomics (repositionable grip, programmable AF stop/start and now tilting screens are on the way).
I think you can find what you're looking for in medium format, as long as the extra cost, weight, lower convenience and performance are worth the improvement in image quality to you. It's a very personal and subjective decision, but if you find a solution that pleases you, the small format solutions won't ever have the same appeal again. (I can attest to that!
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And I almost forgot--welcome! I hope we can help you to find the right setup.
Take care,
Brad
