Corvettes are better. Simple upgrading - by FS2 time cruisers are essentially ineffective cannon-fodder, even a fighter can take them down. FS1 they were useful as there were (initially) no shields, lighter bombing weapons and no beams.
I hear a lot (or maybe just one person a lot of times) saying that FS2 cruisers can be taken down by "a fighter". Am I the only person that played
Feint! Parry! Riposte! were the player is in a Perseus and has to lure fighters from a pair of Fenris back to his own Leviathan. I'm not sure about anyone else, but if I got too close to those Cruisers I got cut apart. Even an Aten with its two AAAfs is going to be a hard nut for a single fighter to crack (unless you have broken Trebuchets).
A lot of people probably rather remember the first mission with the Colossus where a Leviathan was attacking an Arcadia and it was essentially fodder because its four AAAfs were beam-locked. If those beams weren't locked, Alpha 1 would be the only survivor (if that).
Despite their relative fragility, Cruisers can be very effective Anti-fighter elements (especially the Aeolus). Shivan Cruisers even can be very effective anti-ship elements . . . hell a Lilith can rip through a Deimos with ease.
And not every battle is the Colossus versus the Shivan armada. When the Shivans aren't threatening extinction, there are pirates and other small battles were cruisers guarding convoys and the like are very effective. Whose going to get a Corvette to guard a couple of freighters? But a Cruiser, sure sounds good.
As for expense, there is no clear explanation in FS2 as to what is more expensive or not. Any ship mass produced will be less expensive than a ship on a limited run like the Aeolus. Its also why the Iceni was probably several times more expensive than a Deimos.