I just wondered if it was best to concentrate on one skill tree i.e Swordsmanship/Magic instead of spreading the points out between the two?

As long as you don't concentrate solely on Alchemy, you should be fine.

Spreading out is a valid and efficient tactic, especially if you've already finished the game once.

No. Try to get all the Mutagen slots and go from impregnation too.

Personal preference i think. There is no correct way of spending your points, its all up to how you want to play the game.

Personally i'm putting most of my points into Swordsmanship and a few in Magic also. But i haven't spent a single point in Alchemy yet. And i have been doing fine so far.

After 2 playthroughs I realized that spending 3 extra points to get lvl2 impregnation is a waste of points.

But that's just my opinion.

The only worthwhile skill in the Alchemy tree is the one that enhances bonuses and reduces penalties for potions.

I just like getting Impreg so I can attain 100% Crit Rate. Makes killing everything stupidly easy.

That's a valid tactic indeed. Requires extreme spreading out though.

Well it requires you go fairly deep into Swords and Alchemy, but you end up with about 5/6 points spare to spend where you please.

I've been going heavily into swordsmanship, and im trying to decide how far to go into magic. I kinda want the next level of Quen, since I use signs so much, but I want the vigor i've already put into the swords tree. Choices, choices.

Question: Does impregnation add directly to your effects or does it add via percentage? For example if it says add 15%, do you get 3%+15% for your crit. effect mutagen or do you get 3%*1.15?

The percentage is wrong. Lvl 2 (35%) basically doubles the effect of most mutagens (and sometimes more).