Too many variables to make a "blanket statement". I am in the middle of swapping in a v8 myself and decided on a quadrajet. I was planning on FI but came across a deal I couldn't turn down on a low mile 330hp 350 engine crate (vortec heads). I am a roller LT4 hot cam away from 370hp. The problem with TBI, is that the max hp is around 250 or so before you start having issues. The computer cannot be programmed like an OBDII, thus you need to burn chips (go read on the 3rd gen F-body forums). So if you plan to never touch it again or get the chip burning material (costly) it could be an option. Add to the cost the amount a decent intake for TBI and a 454 throttle body to flow enough for the 330hp... I could have done a stock 350 TBI for the same cost as my carb set-up, but I would have left approx 80 hp on the table with the easy potential for more and I WAS NOT going to do that!
The stock OBDII FI unit for 350 vortecs are high pressure (higher than "normal" FI) with poppet style injectors that can have issues. To me not worth the hassle. I saw the only option as a build it myself FI unit. Again, I have the knowledge/skills but to me it was not worth the cost for the gains over a quadrajet. It has been years since I have rebuilt a carb so it may have been easier to build a FI unit as I have toyed around with Mustangs in recent history. To be honest, I had fun rebuilding the quadrajet as it brought back memories and there is a "cool" factor for me.
If I had a vehicle not shaped like a brick that could use the efficiency for more than 1-2 mpg gain, if I had plans to supercharge/turbocharge, or if I was going to constantly be at high angles I would have been more influenced to FI.
If you do not have plans to go above 250hp or so and not a vortec style head, TBI is an option. I would even consider port FI off a 5.7 corvette, Camaro, or TA.
1995 4Cyl Wrangler - Used to having modified Mustangs, I have to get this thing more guts!