Hi,
 
I'm looking to build a new DAW, but I'm quite on a tight budget. I was eyeing a Xeon 1231v3, which is lower clocked but has multithreading. Or should I go for the i5 4690? I don't  have 100 euros to spare for an i7 4790.

Having no experience with Xeon, I'll just do some comparison between i5 and i7.

If I google "i5 4690 vs i7 4790" the i5 don't seems to be lacking too much.
 
And if I compare the i5 4690 with my existing i7 2600k cpu, the i5 seems better on bench mark. So, I think the i5 4690 would be fine.

With Xeon chips being optimized for servers, I'd probably stick with one optimized for desktop. This is supposition on my part, but I'm assuming that you'll find greater software and audio i/f compatibility with an i5 or i7.

Xeon is also targeted at the pro workstation market, CAD, video, etc. I do believe Apple ships or once shipped Xeon-based systems. Could be wrong about that.

Generally speaking, the i5 (or even better/more popular the i7) is what's been favoured in DAW's and the price is not much higher (~$20) for the i5.  

I know the difference between the Xeon and i5 isn't that high in price. But the Xeon has multithreading.

SEVerstraten
I know the difference between the Xeon and i5 isn't that high in price. But the Xeon has multithreading.

Seems like you've answered your own question.

Google "xeon vs i5/i7 for daw" and you may find some answer.. Some says no difference, some says xeon needs to run in pair to see any benefits, but since you're running on a budget, do get the full picture for the total cost (motherboard and memory) and go from there. My system was build a few years back (for running X1).. any cpu better than mine should be fine, as my CPU never hits more than 25% even on my heaviest projects (around 20 plus tracks).
 
From reading this forum, seems Windows 10 (when it settles down and your audio driver is available) may be a better thing to look forward for DAW performance.