There seems to be more selection (at least advirtised selection)now than there was 2 years ago. I looked at Tornado and ERA at the time, both via internet. I really liked the monocoque chassis on the ERA, so went to visit them. Once I saw and set in one of their cars, my mind was made up.
My ERA is scheduled to build first of the year. As with all products, there seem to be pros and cons to each manufacturer.
Pros with the ERA are a stainless steel monocoque and attention to details to make the car as authentic as possible. Details and quality on the cars I've seen are impressive. They are based in the US which was a plus to me. They are really nice folks to deal with. They have been in the business for a long time and have made a small fleet of GT's. They will do a turnkey for you if you wish.
The biggest con is that an ERA will (depending on the options you want), cost more to complete than most others.I beleive their price is now about $55,000 plus options (AC, leather, sway bars, etc). To complete the car, you supply engine, transaxle, brake calipers and rotors, coilovers, rack and pinion for steering, halfshafts, wheels & tires, paint, plus some other misc. stuff.
Toss ups are the use of only ZF transaxles and the 2 year wait. If you want authentic, the ZF comes closest at the present time...at a price. Figure $7,000-$8,000 for a NOS unit if you can find one, plus the cost of "flipping it over" to fit in the GT. $11,000 for a new one. The cost for this gearbox is many many times more than the cost for the Audi gearboxes others are using.
The 2 year wait fit my timetable so it wasn't an issue to me. Gave me time to gather needed misc. parts, do the engine and transaxle, etc.
Bottom line is how authentic you want the details to be, how much money you're willing to invest, how long you're willing to wait, and how much of the work you can and want to do yourself.
Dave