You're going to run out of traction before power (I have R-4s). I've seen posts re. the addition of 20-25% effective traction with FWD (some confuse with power - but it's traction first - power without traction means ZIP). Now, the 20-25% is a high estimate, in my opinion. You can lock (posi-traction you call it - I would differ - it's locking the rear differential on most tractors - don't even think about turning when your rear wheels are locked). Anyhow, with the rear differential, with much larger tires, locked, how much you achieve by the non-locking addition of the front wheels (which means the front wheel with the better traction, not both wheels) is a question I haven't seen answered with data. That said, I paid the extra for FWD, and it's saved my bacon on occasion. I don't use FWD for most work, but when you need it, say with a loaded FEL (and, hence, lots of weight on the front wheels), it sure is handy. So, if you can afford it - FWD, and if not, OK, you'll still enjoy your tractor, a lot, as we all do.