Having both and liking both I think I can give you a fair analysis. I think Metal Gear Solid 5 has better combat but a mediocre story. Witcher's combat isn't that great and can feel repetitive. The combat reminds me a little bit of Dark Souls and Assassins Creed except those games do it much better and I guess that's my problem. I wish you could grapple guys and throw them, use bows, bash people with shields, and have a little more variety than just weak attack and strong attack. However I think the story, quests, and variety of the world is far better than MGS5. For example in Witcher there are regular towns people, even children, doing their day to day work. The world feels a lot more alive than MGS5 where the world is entirely filled with bad guys. The quest system in Witcher is also probably the best I have ever seen in a video game. There are usually multiple ways of completing quests and quests can usually be done out of order. It reminds me very much of the quest structure of Fallout 1 and 2.
Both games do suffer from a bit of repetitiveness however Witcher has a ton more unique quests whereas MGS5 tends to recycle areas and quests.
I personally would recommend both games, but if you must pick one I would choose based on whether you want better story/quests (Witcher) or deeper combat (MGS5). Another thing to consider is whether you feel like shooting guns or wielding swords and magic. i.e. Game based in reality or game that is fantasy.
If I were you I would buy one now and wait for a holiday steam sale and get the other.