Emerald if you like to have lots of guns (at the sacrifice of how much you can AW on it). it's also one of the better UK ships for AA play if you go into GB games with it, with it's 7x R slots.
the downside of the Emerald, outside of it's lack of ability to take much heavy punishment, is if you're using Auto FCS, instead of the manual Aiming FCS, you'll almost always having 1 or 2 guns going wonky on you, and you be constantly at risk of friendly fire as they shoot off in oddball directions but the one the rest of your guns are shooting at (and causing you an exp penalty along with it. and probably some insults from players who'll think you're a noob for shooting at them *shrugs*)
the Dido, you can go heavy AW on it, it's slower than the Emerald even with normal .2 deck & .2 belt, with some bulge & bulkhead added in for protection but not overdoing it.
on the other hand, you get 4x R slots, and can pack quite the punch with the 6" duals, or even the 5.25" MK II duals.
the Dido, at level 34 I believe, you can remodel into the Sirius, which gives you a 5th R slot for added punch. quite a number of people take the Sirius Remodel, go front turrets only, then AW it to the max. as you've probably run against a few times.
downside of the Dido/Sirius? it's speed, and that's not even counting if you go full out AW with it. but the armour & damage dealt do make up for the difference if you play her smart.
in the end, it depends on your playing style, and what you want to use her for. or how well you can adapt coming out of a much faster Destroyer ship.