I personally find the diamond to be pretty useless. The weapon arcs, while basically the same as every other borg ship, are still worse somehow. The sphere/prime is probably the best in the borg fleet, as long as you upgrade the engines and trusters and such.
My problem with the borg is that the lack of shields means the hit and runs can wack your warp core out, so i tend to use everything i have to hurt them, tractor beam, weapons, shuttles, the lot, until i destroy their transporter enough times to stop them from repairing it. once that's done, it's usually easy pickings, unless they wreck your armor too quickly, but it hasn't happened to me much...