Do em both. They're really pretty similar, upper mish may be a tiny bit harder, if at all. Upper mish has more rapids for the length of the run. Low rustic has great scenery in a cool setting, really fun rapids, more flat water since it's a longer run but still has bouncy water inbetween the named rapids, class iv-.
Low'r rustic really starts with Granpas's gorge-a really fun, scenic class iv-. Death by decapitation...or is it dismemberment is next, a good 6or 7ft drop wave thing. It's the one under the bridge. It gets the name cause the wave wants to kick you right into an awkward blunt object sticking out of the bridge. Start left up top with a slighty right ferry angle into the wave and you'll be fine...it boats easier than it looks, plus it's pool drop. Fun rapids follow after that. The last one is 99hole/double drop (mile99). Stay Right of the holes at the top of this rapid. I believe there is still a log submerged that probably doesn't interest you. Again, it's not hard to avoid at all
I love em both. Do them both if you can. Hitchin is a little harder up there, sometimes we bring a bike. Shouldn't be a problem though if it's as busy this weekend as it was last weekend. Sorry, prolly more info than you needed, hope it helps.