As long as its a totally manual lens, you only have to deal with the somewhat klutzy adapters. If I were buying a new lens and it was available in Canon mount, thats the way to go.
However, in terms of being able to resell a manual lens, having one that could be adapted fit Most DSLR bodies might have the most value.
Not only can a Nikon F lens be adapted to Canon, but also to Sony/Minolta, and a few others. That increases the possible number of users who might want to buy it. Just watch out, many of those old Nikon lenses have odd protrusions or other gotchas.
I'll weigh in here, because i've got more lenses via adapter than normal EF mount lenses.
Firstly, my Samyang 35/1.4 (and other Samyangs too). Purely mechanical, there's no linkage at all. Sometimes I wish i'd gotten it in F-mount, then put an F-EF adapter with chip on it. Instead, I bought an AF confirm chip, and glued it on myself. It's not the best glue-job, so sometimes it's a bit iffy, but I can work with it. No downsides to going with adapters, except for what I'll write at the bottom.
Now, Z-E Zeiss lenses. I don't have any (i wish), but as far as I know they're not purely mechanical. Do they not use electronic aperture? Or at least have focus confirm / EXIF? That puts them a step above Samyangs if they do, so in that case using an adapter would be a disadvantage.
As to Spokane's comment about resell being higher on more adaptable (ie F-mount) lenses, that's true in theory. The theory only works if everyone knows about adapters, and I'm pretty sure not everyone does. I've actually noticed in practice, some ebay auctions (for ZE / ZF) glass, the ZF goes for less than the ZE. (although Leica R is priced well above the rest, that's more because of the 'Leica' badge than the long-flange distance).
Also, afaik there isn't an Alpha - EF adapter. Or if there is, i haven't heard about it (or probably you meant F goes on Alpha, not Alpha on EF?)
I've got OM lenses on one adapter (tokina 17/3.5, two 28/2.8s, and a 50mm macro, plus the OM bellows set).
I've got a PK lens (MIR 20/2.5, but when I use the rear screw-in colour filters for B+W it hits my EOS 3 mirror), not as nice because it's hard to get off, need one adapter per lens.
M42 i've got heaps of Takumars and otherwise, one thing with them is that the lens is never up the right way (except for Teles on which you can rotate the mount).
And Medium Format, anything *can* adapt to EF, but some mounts are more popular than others (like Pentacon 6, Hassy 500/2000, pentax 67, Mamiya 645. Not as popular are Contax 645 and Rollei 600*, they have electronic stop-down like EF, so on adapters they're wide-open only. And RB/RZ 67 and Rollei SL66 don't have focussing helicoids in the lens, so infinity or macro-bellows only, or a Zoerk adapter is $1k.
Anyway, that's just me blabbing on, seeing as the OP is mainly talking about Samyangs and Zeiss.
The one main problem I have, with all of my adapters and all of my lenses....
Maybe that's because I buy cheap $5 chinese adapters, but mostly it's just manufacturing tolerances, or wear-and-tear on old lenses. Mostly it's not even the adapter's fault, the same adapter works fine on a different lens, which wobbles with a different adapter. This doesn't affect sometimes in real life, sometimes (like uberfast lenses), it can make the whole image blurry, or one side of it (in extreme wobbliness you get a bit of Schleimpflug happening).
The exception to this is EdMika's adapter on my FL55/1.2. But then, that's not a 'real' adapter like the rest are. No offence Ed, what I mean is that it's better, it's more a 'back mount replacement' than 'adapter'. It's solid as a brick, no wobbling whatsoever...
So, at the end of all that. If you're planning on jumping ship, or just want to keep options open (or even like me, you just want to buy the cheapest no matter what mount), then go with an adapter and F mount, as long as you know that it might not be perfect. But afaik, at least zeiss ZE lenses probably work better natively...