MacbookPro 13 or 15 2010 for software development
It's a little tricky to see what your question is here so I'll give my views but my short answer is that 15 inch will be a better experience but not in the sense that a 13" MBP could not do what you need at all.

Dual-boot will be fine although I find OSX much more efficient in terms of resource needs. You'll have no problems doing web development using either of those two frameworks on either OS.

As for mobility, the weight and size difference seems negligible when carrying it around but once it is sat down in front of you, the extra screen space is useful.