It all depends on the power level of the engine and the car it's in.
You are going to get a lot of opinions from both sides, so be ready for it!
So, here's my stab at it FWIW.
Instead of shorties, why not long tube?
If you increase your power level sometime in the future, you will wish you had done it right the first time.
Unless you have them already, good 340 manifolds are pretty expensive and the TTI headers would be about double the cost, but you are potentially gaining another 30 to 40 horse with them overall.
This is taking for granted that you have a 350 to 400 H.P. small block with decent flowing heads that spins to 6,500 R.P.M. regularly.
If you have a stock 318, then stock cast iron manifolds are just fine with dual exhaust.
On a side note, the F.A.S.T. cars run cast iron exhaust manifolds and stock size bias ply tires and they make loads of power and e.t. very well, so nobody can say it can't be done that way, but there is a lot of science that goes along with it that most people don't know about like striker cranks, perfect machining, balanced and blueprinted to the max, chassis perfectly set up, etc..........
In the long run, do what suits you and your long term expectations.