I have a solution that I have done many times: do both! The hike up Mt. Frissell from Mt. Washington Road is actually pretty short, about 2 miles one way, really a very brief jaunt. I would suggest starting at the Undermountain Trail on Rte. 41, go up to the AT, turn right and go over Bear Mt., then down the steep north side. Now is the slightly tricky part. After the steep part, you then traverse across a slope at a much less steep grade and finally drop to a pretty flat section where you will come to a raised rock "bridge" over a swampy section (not so this time of year). Here there is a very distinct but unmarked trail on the left that goes to the Mt. Washington Road (past the AMC Northwest Cabin). You will know you missed this turn off if you continue on the AT and get to the Paradise Lane Junction.
Once at the Mt. Washington Road, take a right and quick left on the trail to Frissell. Shortly after you leave the road, you have to go left on the red blazed trail. You climb gradually then very steeply up over Round Mountain, a very pleasant, open, rocky summit on the way to Frissell. Then you go down a bit and very steeply up Frissell. Not many views from the top. But continue over the top and down a bit to the CT high point, also some very nice spots to sit on open ledges on the way there. From Undermountain Trailhead to this point is about 6 miles. If you want more you can keep going to the tri state marker (NY, CT, MA) in another 1/3 mile, and then to Mt. Brace (another mile and a half). But remember you will have to go back!
On the way back, retrace your steps, but when you get back to the AT, you can go left and then right on Paradise Lane back to the Undermountain trail, 1 mile from route 41 (avoiding going back over Bear Mt. again).
All and all about a 12-13 mile hike, without the added distance to the tristate marker or Mt. Brace.
Any more questions, please ask!