Moltres > Entei
That's my opinion and I want to explain, why.
(FYI, English is not my first language)
Of course, Moltres has double weakness to Rock. Still I see no problem there, as it's not recommendable to use one of them against Rock anyway. Moltres has a better attack stat (and defense), but for offensive mons, I prefer a higher attack stat.
TL;DR I think Moltres is overall better. What's your opinion? Is one of my points wrong? Do you have something to add?
Answers
I don't understand the point when you say that Moltres is going to be better than Entei against Metagross. Maybe it will get something like Hammer Arm which gives to Moltres a good advantage comparatively to Entei, but it's not sure.
I think Moltres could be a bit better than Entei, but I will probably get one of each at level 39.
Nice analysis! Also even if you want a specialist against ice, the most relevant is against Articuno, since neither is particularly good against Lapras (or any other ice that might have water moves). So that further narrows the field of likely opponents to Bug/Grass/Steel, favoring Moltres' double-resistance in 2 of the 3 cases.