I've got an 06 Foreman air cooled. Have never had a problem at all with covering it in mud I can tell you that much.
I've rode on the Rubicons, I don't know how much the liquid cooling makes a difference, but what I'd look at is this.
My foreman feels peppier than a Rubi of the same year because of the transmission, I've got no problem shifting manually so the Hondamatic on the Rubi made no difference to me, even on the manual setting on the Rubi it feels sluggish compared to mine though it does shift smoother because of the hydraulic transmission.
On the Rubi though you do have low range which the foreman does not if that makes a difference to you.
Also realize the foreman is actually 475cc, the Rubi is 499.
I personally didn't see a point in paying extra for the Rubicon for features I don't need and that actually made it less fun to ride. YMMV
Try to drive both before you make a decision.
If you lived anywhere near here I'd set you up to try both but alas, Texas and Virginia are not quite neighbors