I think these are the main issues:
Money- Depending on the price of a tent you are interested, can't say which is more expensive. Depends on size and the materials you have available and wish to use.
Work- Tent is definitly much less work to start up. Just slap your parts into the pre existing structure and start growing.
Layout- DIY will allow you to make any layout you like. Make venting go in and out from any spot you want, as well as everything else from electrical, water sources etc.
I use a "room" and I have made so many changes to it from new ideas, and old ideas that didn't work. If I had a tent it would probably be in tatters now from all the removing and rebuilding I've done.
BTW I vote for building your own room. Mylar is optional, I'm partial to painting it white, but Mylar is great.