Tough Decision. They are completely different rivers.
The Pit has murky, warm water, deserty-volcanic scenery, is Class 4, with a big waterfall, and has some great rapids that do keep you busy and entertained. Pretty easy shuttle, too.
The McCloud (and I'm assuming you are talking about the Upper (Hearst) run. The lower McCloud is an overnighter and is getting a bit lowish this year.) has beautiful, old-growth scenery, crystal clear, ice-cold water, and starts out from Lower Falls with minimal water, often requiring getting in & out of the boats to shove through the volcanic rock dams (I've even seen it with no water at all at the put-in, despite having water flowing at the falls, the water returned downstream). Once you get to Big Springs, it's basically one big long (8 mile?) rapid that's mostly class 2, with the Class 3 part just below Big Springs. Lots of rocks to hit. Then, of course there's the Hearst Estate which is pretty incredible (just don't get out of the boats to look, keep floatin'). Lastly, there's the 3 miles out on the Lake to the boat ramp. Oh, and take your drytop and gloves, that water is cold!
The McCloud is one of my favorite rivers to take people (who are in decent shape, hitting all of the rocks on the top part can take it out of you when you ain't a good boater) who are new to the sport, but can boat Class 2-3 stuff (in a kayak you want to have a good roll, wouldn't be a good place to swim; shallow and sharp rocks) just because of the beautiful water (it glows, almost) and the scenery. I haven't done it in the last few years; the shuttle is long, and there's the paddle out on the lake, so it's more of an epic.
The Pit is way more of a pool and drop deal with rapids easily a notch up from the McCloud. Great Scenery, too (just different). The toughest part of the Pit is the beginning, when you work your way down through the brush hoping you found the right slot.
I'll be on the Pit, just because it's flowing. But, I live in Redding, so doing the McCloud can be done anytime.