I took a 14'er down in March. My advice? No problem with the boat, but don't get too cocky. I made it through all the big rapids no problem, but flipped in a rapid no one has ever heard of thinking, "I can take a piece of that hole...". You can get away with almost anything down there in the bigger boats, you need to be very careful with the smaller boats.
Having only rowed 18-footers down there before, I really enjoyed the smaller boat. It was a lot of fun in some of the big stuff - the waves in Hermit probably being the best. Everything seemed so much bigger in a little boat that I ever remember it being.
Don't underestimate the amount of gear you have. I have never been on a trip where we are loading at Lees Ferry and had lots of excess space. You can probably carry 3 or 4 times more gear on an 18-footer than a 14-footer.
If you do rent an 18er from Brady or Pro or someone like that, you likely won't regret it, they are really solid, good boats which rig up quickly and carry a ton of stuff.
Oh yeah - don't go left in bedrock.