First off, that's going to be a great hike. I spent quite a bit of time out there last summer and it's a beautiful area. If it were me, I'd lean towards a warmer bag - the 20 degree is a bit much but if I had to choose between that and a fleece thing I'd go with the 20 degree. It can chilly at night up there. I used my 32 degree bag throughout the summer and was comfortable. If you're at 20lbs with the 20 degree bag I'd say let it ride brother, that's a nice pack weight! You'll probably end up a little toasty but that's better than shivering in my book, and I think that's what will go down if you take the fleece bag. Just my two cents.
Feel free to continue the tradition some friends and I had of drinking a can of Modus Hoperandi on top of every Fourteener we summited, including Holy Cross. Nothing like a super-hoppy brew at 9:30 am on top of a mountain. I daresay I jumped for joy.
Best of luck and have a great time. I'm jealous!