Mike,
As Ashley says, this essentially boils down to "yer pays yer money and yer takes yer chances" … or in other words what kind of model builder you are, and in consequence what you are likely to take more pleasure from: getting one model just so, or getting to build two different ones.
For what it's worth (not much!), for my part I tend to favour spending my time and money on fewer models rather than more, and so far I have only ever had one project on the go at a time. Also, to my mind the fact that you intend to build a static model argues in favour of the photo-etch. If your Snowberry were to be converted to r/c and go on the water, it would perhaps be more important to aim for maximum sturdiness and consider the fact that most of the time that counts, the model would only be seen at quite a distance. However, a static build is likely to be most often looked at from much closer by, where the added detail (and quality of detail) will make much more of a difference; the model will also be much more protected (perhaps even in a glass case?) or at least much less likely to be knocked about.
My personal experience of photo-etch is limited, but I have been at it enough to know that it can be frightfully fiddly, but also that the result, if worked with reasonable care and attention, can be really lovely, and that it allows for much finer and better-to-scale detail than styrene plastic.
Again, whether or not it'll be worth it for you, nobody but yourself can say.
Now, I am not at all up to scratch on which updates exist, in which bundles or combinations, but if you were to go for the middle road option of getting the Fairmile and just some of the photo etch, I'd say that to my mind, top candidates for replacement on that type of vessel would be things like railings and stanchions, ladders, companionways and smaller caliber armament.
I'm sure you have already done this, but if not, I would also suggest two further lines of inquiry, namely to image Google the matter, and to trawl the various plastic modelling fora out there. In the latter case, I'd of course suggest to look for the kit in question, with and without photo etch, but also in general at what the plastic crowd does with photo etch on e.g. tanks and suchlike — this'll give you much more of an idea what sort of difference it can make, and might help you better understand how much (or little) that approach appeals to you?!
Mattias
Edited By Banjoman on 18/06/2018 06:34:59
Edited By Banjoman on 18/06/2018 06:36:24