I would start A as he's better in both defense and offense. Game shape difference is not that big of a factor I think, A could easily reach B's game shape in 2-3 weeks. Besides, I don't think B's game shape would make him a significantly better player than A. Also, even if you are training bigs, managing their minutes won't be too difficult since they're pretty close in terms of salary and have poor stamina, so B will also get some good minutes off the bench.