If there's not much in the temps then you should put more emphasis on:
1) ease of application: e.g. LC pro which dissolves aluminium on contact and is very hard to clean off if it gets in grooves of circuit board looses lots of marks here.
2) how long it lasts: e.g. all the "silver" based pastes start to loose efficiency after a year or two and hence should be avoided unless you constantly change your cpu.