Personally I would hire one to have a go, that's what I did before I bought mine, lensesforhire have the Mk1 for £49 for 3 days and the Mk2 for £69, it might be worth seeing if they are doing any holiday deals (
www.lensesforhire.co.uk)
I have had a quick look at the Samyang specs and I cannot see if it can tilt and shift in the same plane like the Canon Mk2 .Having had both the Mk1 and now the Mk2 that ability which is not present in the Mk1 ( without dismantling the lens) is very valuable, you don't need it very often but when you do it makes a world of difference.
Using a TS-E is a different mindset in that it really slows you down and makes you think about your shot which is no bad thing.
Personally given your camera if having tried one you liked the concept of using a TS-E lens I would save up a bit more ( well a lot more) and get the Canon 24mm Mk2 , you should be able to pick up a used one for around £1K, really if you are going to get a TS-E you might as well get the best.
Another thought I know it is very expensive and the rarely appear used but the 17mm is worth considering , it takes a TC very well and does give great flexibilty albeit at a price.
If you do decide to get one whichever way have a look here (
http://www.northlight-images.co.uk/article_pages/using_tilt.html) its quite illuminating.
Best of Luck whichever way you go