I think thaty if you're planning to go to the UK, you should take the IELTS. Most universities say they accept Toefl, but prefer the IELTS. Now, if you focus is US, than it's the other way around.

If you only apply for UK universities, I would take the IELTS test. Just because they "seem" to prefer it.

Should you on the other hand also apply to US universities, I wouldn't hesitate to take the TOEFL test. If the UK university wants to accept you for their program, they'll be glad once you have completed any language requirement, be it TOEFL or IELTS.

The difference in reality is probably inexisting.

It doesn't matter - they know how to read them both!

I don't think it does matter.
I took TOEFL (never bothered about IELTS) and every University I appleid to had no issues about that (either UK or Netherlands).
In my opinion crucial is the result. If have Your TOEFL well above requirements it'll perfectly fine... it might even look better than having IETLS score just what You're supposed to have...

IELTS is easier than TOEFL. Especially when you consider the high scores UK universities tend to demand. Oxford and Cambridge require more than 110 points out of 120 whereas Harvard, Yale and all the other top US-Universities only want to have more than 100 points with some requirements on certain parts.
I have a few friends who didn't make the 110 at TOEFL but succeeded in achieving the requirements at IELTS. Therefor and because the circumstances at a TOEFL Test are aweful (especially with the speaking) you should definitely opt for the IELTS Test. Having said this, if you apply for UK and US Universities: take the TOEFL!