I've used both on a 7bbl scale and I would say that they both have their pluses and minuses. I used to work on a system that had a direct fire propane kettle and electric HLT (we had very cheap electric prices in that town). The direct fire kettle was great, very vigorous rolling boils and no waiting around for the boil to get up to temperature. Really good carmelization in the kettle too. The electric HLT was crap though. Took 12 hours plus to heat ground water up to 190 which made planning ahead of time a necessity (if you forgot to turn on the HLT before leaving for the night, there was no way you could brew the next day....). I guess the biggest positive was that there was no boiler to deal with. No steam traps. No condensate return pumps etc...
Now I work on a 7 bbl steam jacketed system. I like it just fine. It certainly is nice to be able to heat up water in the HLT very quickly and the boil is strong. My biggest complaint is that I end up waiting for the boil to start for 20-30 minutes after runoff is done, but that's because my bottom jacket doesn't have a separate shut off valve, so I can't turn the jackets on until they're covered by wort. This is a flaw with the installation of the steam piping, not the actual brewhouse itself.
I can't think of a really strong reason to go with direct fire vs. steam. I would say it depends on what kind of brewhouse you get a better deal on, what propane & electrical prices are for your brewery, and whether you have space/desire to have a boiler.
If you're planning on ever brewing smaller batches (like 3-5 bbls of a strong ale in your 7 bbl system) then I would say that the direct fire is a better choice because you won't have to worry about covering the jackets in the kettle.
Head Beer Guy