Now then. This is such a hard question for me to answer and probably will be for some others out there as well.

Lets just go through the history first though.
Kai Hansen leaves Helloween and starts Gamma Ray with Ralf Scheepers
The first three Gamma Ray albums are with Ralf on lead vocals, but then they decided Ralf should leave because he lived the other side of the country and so wasnt easy for the band to practise, etc.
Gamma Ray then goes on with Kai taking lead vocals and has made five more albums now like that.

Ralf out of Gamma Ray auditions for the position of new vocalist for Judas Priest - but doesnt quite get it losing out to Tim Owens - he did end up in the final three though so he did a good job.
So then Primal Fear is formed and have currently released six albums now.

Now to answering the question.
I have known and been into Gamma Ray much much longer then i have been Primal Fear
I cannot pick one best Gamma Ray album or say any area is better because i love 'Heading For Tomorrow' like i do 'No World Order'
Primal Fear have two albums less then Gamma Ray - but six is still quite a meaty number
its really hard to say which Primal Fear album is my favorite as well, but it is probably either 'Primal Fear' 'Nuclear Fire' or 'Seven Seals'

Primal Fear sound a lot like Judas Priest, especially the 'Painkiller' and 'Jugulator' albums and so have a lot of people going around just saying they are 'ripping Judas Priest off' and such - but their last two albums have been quite different and shows clearly they can make good music without sounding like another band

This is so hard though, I have known Gamma Ray longer then most other power metal bands I know, but I just havent listened to them much latley at all (apart from now, I have 'No World Order' on)
But I have been listening to Primal Fear a lot, and have been for a while now.
I spent so much money on my Gamma Ray albums as well - I got them at such an expensive price at £18 each - thats like 37 US dollars each for people who dont use pound sterling

I suppose I am going to have to plump for one though arent I - I cant really make a poll and not answer it myself.
When I started making this I think I was going to vote for Primal Fear, but I actually feel like voting for Gamma Ray now, so I will. purely because I have known them longer I guess.

But oh well - I am going to be very interested in what everyone else thinks!