TAL Differences

Still hunting for an ideal scope, its a minefield- anyway, I have found
Beacon Hill offering reasonable prices on the TAL range which sound okay,
but I'm a little unsure as to the differences between the 2M and the
150PM... one is f/8 and the other f/5 I believe, and from my photography
knowledge this would suggest the f/5 would give a brighter image? I'm sure
it cant be that simple, would the image be brighter but the quality worse?
Is one going to be better for some types of viewing than others? I found a
review of the 2M that said the image quality is very good, can't find much
on the other though....
Thanks
James