I don't think we know enough about the big ships to make good decisions about them yet, but my guess is the Idris will require about twice as many players as the Polaris. So to me the question comes, which are you better off with, a single Idris M or a pair of Polaris? I think the pair of Polaris may well be a better deal for two reasons. First, it will be so much easier to crew one Polaris more often than the Idris far less often. Second, the Polaris is likely to be far faster, so able to be in the thick of fun while the Idris is poking along wishing it were having fun.
History has many examples of pairs of Corvettes engaging single, much larger Frigates and coming out victorious, and this for many reasons. If we find in game that speed matters as much in the 'Verse as it does in real life, my money would be on the Polaris or a pair of them.
And on a side note, as cool as the Zestroyer rail gun will be, without real stats my guess is, 8 S10 torpedo launchers will be far more effective. A pair of Polaris has much more firepower than an Idris M, but will not be able to take hits the same. If the Polaris can evade ship-to-ship weapons like the Zestroyer, it can rule over the Idris M.