Too many variables to be sure, but as others have said - longer waterline is almost always better. I don’t think the momentum/weight of the boat would be a significant factor at all (and if anything I’d bet on a lighter boat, though the difference would be tiny).
I’d expect the biggest factor between the two longer boats is how comfortable you are, and how able you are to maintain an efficient forward stroke (for example, if one feels “tippy” and you are bracing rather than paddling, or one feels very wide, so you cannot plant the paddle and pull it straight back - neither of these cases are likely for you - just suggesting how ergonomics might mean the “best” boat for you isn’t the best for someone else). Good luck and have fun!
Water conditions? Some are better in flat water and others better in the chop of open water. I have seen within a distance run as water conditions change so does the lead boat. A dragster is great on a 1/4 mile track but maybe not the best for a pikes peak run.
A “heavier” boat will only give you momentum when going down a steep, long waterfall. It will ride deeper and be slower everywhere else. Pick a course and time yourself in each of these boats. That is the only way to know for sure. Boats aren’t fast, per se. They are only fast when matched with an engine that knows how to unleash their speed and be comfortable in the boat. In general, the longer, narrower and lighter the faster.
With a rockered hull the Elaho is more designed for maneuverability than tracking and speed. Of course, the Gulfstream has a rockered hull as well. Whether it is fast enough for you will depend on your ability, the class you are put into, and possibly conditions on the day of the race. When is the race? Let us know how it turns out.
