If you're running 2014 on 6Gb, you'll probably see similar performance on 2015.
For sure, one thing you don't want is to run out of while using SolidWorks is RAM. The second Windows starts paging memory, you're SolidWorks process is forfeit.
Having said that, I don't think I've ever seen the performance graph go above 8Gb on my machine for something that wasn't a horrible memory leak.
Generally, RAM is so cheap these days, that there's no compelling reason not to have 16Gb in your machine. 2 8Gb DDR3 modules cost about $250.