If you are attracted by the cosmetic side of the new 500, which I think you are..save some money and get the new one. Otherwise, keep the 350 and keep riding.
But then again, if your heart is really set on the 500, then sell the 350, save some dough, get the 500. Or as Scottie J said, get both! In the end, keep whatever puts a smile on your face. But sometimes in the case of an Enfield, two different sets of 'models' bring the same smile. So it is very hard to decide sometimes..