Budget is absolutely the most important thing here. For smaller stock like picture frames, I'd forgo a slider. More spots that "move" the more opportunities for failure exist. As people mentioned, proper setup and technique are likely going to be the most important.
I haven't touched a kapex unfortunately, but the best miter saw I've personally used has been the hitachi C12. has a very tall, very flat fence, great base, power and smooth operation, just a great tool. The positive locks were fantastic as well. Was a real bummer when I had to give it back to my friend lol. That said, I own a dewalt DWS716, does what I need it to do without many complaints. The lock system isnt as good as the hitachi, and the wing extensions werent that great, but thats a project for another day.