If I had only those two to chose from, it would be the 340, even if I had to put all teener parts on it. I once built a combo that way, and a 340 with a teener cam and 2bbl top-end is still stronger than a straight teener. I called it my big-bore teener.
But If I was limited to any small block, it would be the 360. The extra stroke allows a little more cam before the Dcr falls off. Or cam for cam; more everything,including mpgs.I would pay extra for the 360, and you couldn't give me a teener.In a well-built DD with a small cam, you will save the cost of the 360 core in better mpgs alone, probably in the first year;certainly in the second.
But in a streeter, I wouldn't be in a hurry to put a big cam in it. Cruising at sub-1600rpm with an O/D, and ripping off low 12s, all while getting hi 20's mpgUS,( I got up to 32USg),is kindof intoxicating. Just try that with a teener....not even two out of three.
Your vert will thank you for the extra cubes.