Spigot

I think your first question should be how often you are anticipating using your bike for commuting, and how often for mtbing. I was in the same position as you 6 years ago, and I bought a mtb so that I could use it for commuting and mtbing. Over time, I have been commuting about 3-4 days a week, and I go mtbing about once a year. Given that, I would prefer to now have a dedicated hybrid, because:
- suspension adds weight, reduces efficiency and increases maintenance requirements and cost; and
- hybrids have larger, narrower-profile tires which, I'm told, improve efficency.

If you do go for a mtb, you should DEFINITELY get slicks. Mine are 1.5 inches wide - I don't think you should go any wider than that (they range in width between 1.0 - 2.25 inches). The narrower the tires, the faster the ride, but the more you feel the bumps. I would guess, but don't know, that narrower tires would leave you more susceptible to punctures and buckled wheels.

I bought mud guards (fenders) after my first day riding in the rain. They make a HUGE difference to how wet/dirty you will end up being.

Finally: don't forget to get GOOD lights.

Enjoy the ride!!!