Use a free trial period on FindMyPast to try a test of your tree data in their scheme.
I've just tried a transfer with one of my trees, and found events which were put in the GEDCOM by Ancestry, but thrown away by FindMyPast.
Images, of course, are not transferred, and if you upload them to FindMyPast, there is no way of seeing anything other than a thumbnail, so you need to make sure you have local copies before your Ancestry sub lapses.
There are differences in the way they want you to organise your data; FindMyPast has events like "1871 Census" while Ancestry only has "Residence". However most things should be understandable after the shift. Incidentally, FindMyPast knows the dates of censuses while Ancestry only knows the years.
Having tried both, I think the Ancestry tree software is a bit slicker, but I find them both very clunky when compared with the software I use offline. Both need a huge number of mouse clicks to navigate, but FindMyPast needs more.
FindMyPast have a public/private option on the trees, but at present "Public" just means that they can be accessed by another subscriber to whom you've sent a link - there's no "search public trees".