My advise: KEEP EM BOTH. :cool:
Honestly, they both have their pluses. I've switched to a KTM300 2 stroke and love it. But I still like my Yamaha 250F.
My KTM feels lighter (less top heavy) has a ton more power and I like the suspension better for the desert. (Both bikes have been set up be Pro Action, revalve and springs) The 250F I've now reserved for trail duty. Obviously, your 540 has more power than my 250F. So If you can't keep them both, then you need to decide what you want? Why would you trade the 540? Is there something you don't like about it?
If you do decide to go 2 stroke but like the 4 stroke grunt. I'd strongly recomend a 300. Both KTM and Gas Gas make them. They are not as pipey like the smaller bores and you can lug them if you want. IMO the KTM feels bigger and more spread out than the Gas Gas. The Gas Gas feels smaller and more compact. Also the Gas Gas motor tends to be smoother with less hit than the KTM.
Just to confuse you more, I rode Thumbs KTM 200 at our last outing. If you want a slim and small 125 feeling bike with more power than a typical 125, this bike cannot be beat. It also was awesome to ride.
That's my input, good luck in deciding what you'd like.
Gettign a new bike is always cool :yeehaw: