It's one of those "it depends" questions.
One issue is what size stroker? You can fit a longer stroke and rods in a RB block, but it is not really an issue unless your talking 4.35"+ strokers.
The 4.25" and smaller stroker engine would give the "B" block a bit of an advantage, but I think the differences might only be important when making over 600 HP? If you plan a stroker making near 800+ HP, use an aftermarket block.