I never think of pumpkins as being particularly sweet, However you may find other forms of winter squash more to your liking. Acorns for example are not very sweet so I bake them with a brown sugar, butter and apple filling. Delicatas may also be a possibilty. Cushaws are also milder than most of the orange types.

They say Jarrahdale is not sweet at all but richly flavored. I have seed of a Turkish one which I've shared with many people; it's a large gray one simmilar to Jarrahdale but a bit darker and more irregular in shape, with really deep orange flesh. Not particularly sweet but richly flavored. i can send you seeds too if you like; but write to dolichos@gmail.com.

sazji is right about Jarrahdale, it is a very nice eating sqaush and another squash I grew along with Jarrahdale was Queensland blue, similar but I think I like it a little better, darker flesh....made a nice pie and soup...

Avoid the delicatas - I'd say they are sweeter than pumpkins

If you can find them, Kabocha aren't very sweet and are very good eating.