I prefer NiMH batteries. The reason is that they have excellent re-charge time on the flash. Normal usage, I'll have something like 2 - 3 seconds. If I use Duracell alkaline, the recharge time can take up to 10 seconds.
The biggest downside is that NiMH cannot maintain constant voltage so your flash will fail mysteriously/suddenly even though your NiMH batteries are technically not flat yet.
You can practically drain the alkalines to the last drop.