Mine is the original 660 (same as the Grizzly engine). I'll be trading it in on the newer version with the bigger engine in the spring.
I personally (based on my usage, which is not farm related) see no reason to go for the smaller 450 engine.
Mine is a mini-truck, used mainly for exploring the backcountry. As such, it gets loaded up with food, water, and enough emergency gear in the bed to survive a breakdown. A passenger usually adds anywhere from 150-200 pounds. The 660 easily handles the load, and with the original factory CDI replaced by a Black Rhino CDI we've clocked it at 47 MPH with a full load on a flat dirt road.
It continually amazes me at the types of terrain it can cover, and how well it climbs in 4WD with the differential locked down. Mine got me out of a deep wash on Sunday that I thought I was going to be parking in for quite a while.
I don't know about your finances, but if you can possibly swing it (there shouldn't be too much difference in the monthly payments if you float it over five years like I did) the extra money spent for the full-sized engine will give you the most versatility around the farm and for recreational use.
You'll lose a part of what makes the Rhino what it is if you drop down to the smaller engine.
I've encountered people on the trail who did buy the 450 & were satisfied, but they were older folks who never planned to push it very hard.
The Rhino can be a real workhorse (by far the best wheelbarrow I've ever owned & I've moved a fair amount of dirt with mine), but I can guarantee the bigger engine will work better for log dragging than the little one.
Denis