I agree w all the above except:
Bcowanwheels: I disagree about the mileage (but agree about John 3:16)
I tested a bunch of 4bbl carbs on a stock 250, Nova powerglide 3.08. Overall average MPG for the 4bbl carbs was BETTER than for the Rochester B/BC/BV carbs. Only about 5% or 10% better but still better.
As far as performance, I clocked 60ft-330ft-660ft.
My formula for CFM is "double your horsepower". Thats the CFM you want at 1.5" Hg, the 4bbl style rating. A little less with a heavy car or tall gearing. So for a stock 250, plus 4bbl, plus exhaust, you should have 170-180hp so you want about 350cfm. Only the old Ford Teapot (aka Firebox) and a few others are THAT small. Otherwise, your closest bet is the 390 Holley, the one Tom Lowe noted. If you plan to build a SERIOUS 250cid, its too small, but with just carb, exhaust, maybe small cam, its about perfect.
I also tested a 450 Holley that ran so close to the 390, I'd go with whichever one I got a better deal on.
That said, in my testing, the 600 Edelbrock gave the little Holleys a good run, even besting them in some of the testing. Such a HUGE carb? What I think was happening was, the 600 Ed was just using its primaries and the secondary transition circuit, so it was more like 300cfm total.
In my testing on v8s, the secondary of the AFB will not keep pace with the Holley secondary, it will give up a carlength or two in the 1/4 mile.
I didnt have a 500cfm Edel / AFB to test, that would have been getting part way into the secondaries.
Holley carbs are notorious gas hogs at light throttle, so I would bet on the 500 Ed/AFB for better mileage, but not by much vs the little 390 Holley if jetted right as Tom says.
Get whichever one you score a deal on, 390 or 450 or 500.
