I can speak from some experience, as I've ridden the 350RR, and currently own the 450RR, albeit these are 2012s.
Keep in mind that there can be significant differences year to year as Beta (being a smaller private company) can move swiftly, and isnt afraid to implement significant changes to better their product.
- 2010 was the first year Beta started using their own engines, so this is one of the most significant component evolutions (year to year).
- the 350 was only introduced in (late) 2011, and was significantly revised for 2012 (2014s are now coming !)
- the only place the 350 lacks, is on the tarmac (as I ride to the trails), on the trail, (while there's a minimal weight difference) it feels alot lighter than the 400 !
Some resources, which describe in detail the weight, bore/stroke, improvements, etc...:
http://www.motorcycle-usa.com/840/12484/Motorcycle-Article/2012-Beta-350-RR-Comparison.aspx
http://www.dirtrider.com/tests/off-road-bikes/141_1112_2012_beta_350_rr/
http://www.enduro360.com/2011/11/05/products-tested/2012-beta-350rr-exclusive-first-ride/
