I have the evolution double bevel one. I bought it for a specific job, a whoe load of decking I have now installed. It came with two blades, multi material and wood. I have only used it on wood. Must say it is a great bit of kit, especially for what I bought it for, paid for itself a few times over in terms of time saved. I have only used it for that sort of "rough" job, i.e. decking, not used it for any finer work like skirting, but I think it would cope with it with no problems.
But back to your actual question, the double bevel comes into its own when you have got a 5.4m 39mm decking board into position for a mitre cut and realise you put it the wrong way around, saves turning it over, especially if you would have to turn it end over end. If you only work with shorter more manageable pieces then it is unlikely to be of that much use. But, if you can stretch to the extra it may be worth it.
Do note the saw is a fairly heavy bit of kit. I have a proper mitre saw stand with fold out legs etc., and it is great on that. Wouldn't want to use it on a flimsy top or on the ground though. So do consider what you will want to mount it on and remember the space needed for the sliding head.