Sounds like you made your choice and it is a good choice. If I could do some after the fact weighing in here though, it might help provide more piece of mind. I have experience paddling in in both of these canoes. Both the Grumman and the Charles River have nice lines that will not sap off your speed due to hull shape. That is the good news. In fact the Charles River is one of the most "classic" hull designs and may be one of the best selling canoe hulls of all time (see
http://www.wcha.org/ot_records/models.jpg from the Wooden Canoe Heritage Association's Old Town research project. These are the records for just Old Town. The Charles River was produced by many manufacturers and was a popular model for most of the manufacturers who made the old style wood and canvas canoes. ((Do I score more points for putting in a reference?

)) The Charles River is an old old design (well over 100 years old) that was literally originally derrived from old Native American birchbark canoes. The Charles River reached its heyday in the 1920's as a wood and canvas canoe produced by the Old Town canoe company. The sales were so successful with the Charles River, it was kept when operations were shifted from wood and canvas to more modern materials such as royalex.
I paddled a 17' Grumman in the USCA aluminum nationals last year (I have attached a pic, but please ignore my form as it is awful in that pic - we were at the end of a sprint to the finish line). I was the only person who paddled a Grumman (the rest paddled Alumacrafts). While the Grumman was not as good of a racing canoe as the Alumacrafts (the Grumman appeared to have more rocker, which is not necessarily a bad thing, but not advantageous in the racing conditions in which the nationals race was held), I was impressed by the way the Grumman performed. For an aluminum canoe, the Grumman had some glide, but not as much glide as it would have if it was a lighter canoe. Your 18 footer will have better glide then the 17 footer and it is probably made with less rocker too. This is good for you. As far as balance goes, the Grumman was solid as a rock and I was able to put 100% of my power in each stroke. The 18' Grumman will be a better canoe then the 17' that I trained and raced in last year. In fact, an 18' Grumman is one of the best aluminum boats made and is one of the few aluminum canoes that I would think would perform better then a Charles River. The additional length will provide additional displacement and the lines of the canoe will make the canoe glide better. The added stiffness of the aluminum will also ensure that the maximum power is applied to each stroke. In my opinion, the Grumman would be the better boat due to the added length (when compared to the Charles River) add stiffness of the aluminum. The downside is the additional weight, and the inability to outfit (seat tweaking) the canoe due to the canoe being made of aluminum. One other thing I would be remiss not to add is that THE GRUMMAN WILL HAVE MUCH BETTER PERFORMANCE CAPABILITY WITH A STERN FOOTBRACE. If you do nothing else to the canoe, it is paramount that you put the rear foot brace in. You will dramaticly increase your canoe's performance and maximize the efficiency of each paddle stroke by doing this.
The Charles River is a lighter boat and my knee jerk reaction would be to go with the lighter boat. In fact, when you do time trial comparisons of each canoe, you may find that your comparisons of the Charles River over shorter distances will show that the Charles River is the faster canoe. That will make your decision even more perplexing. As Charlie (Yankee Clipper) points out, royalex sometimes "oilcans" (heck aluminum does too, so you may want to watch it on the other boat as well). Oilcanning is when the bottom of the boat ripples as the boat travels through the water. Oilcanning can sap the speed of the canoe. Also, as a material, royalex flexes in that the boat will tend to twist longitudnally when you are paddling the boat with power. Flexing over a period of time will sap your paddling strength. Finally, while I am not 100% sure on this, I think that the lines of the Grumman are more efficient lines then the Charles River. Over a long haul, efficiency is key.
As West points out, either canoe is a good chioce. They are both good canoes and would both serve you well in the race. So the good news is that while the decision is hard, either way you make your decision, you will most likely not be disappointed.
On a side note, I am doing two races this summer in an aluminum canoe as well. I too chose aluminum over royalex. Right now, my partner and I are in the process of outfitting our canoe. I would be glad to share any info with you in terms of our trials and errors in regards to our experience. The first of the two races is on May 1st. so I will keep you posted (if you would like).
You quiero mas canoe,
Wally "Mastodontico" Werderich
Los Humungos Paddleos. Be afraid, very very afraid...
P.S. What is that polishing coumpound? It looks like it works great.