If you've already bought the Q701, I doubt you'll be disappointed. If you're still mulling it over, I have heard both and can offer some insights into their differences. I tested both before buying the DT880 250ohm, but for the first half of listening I leant towards the Q701.
The Q701 has superb instrument separation and a huge soundstage, although I think the Beyer's imaging is slightly better. Its detail retrieval was about on par with the Beyers, but because of the better instrument separation I found myself discovering more new details in songs vs the DT880. Bass response was definitely weaker than the Beyers, although I understand the original Magni had excellent synergy with the 701s and vastly improved their bass response (it may be worth looking into the Magni/Modi 2 vs the original Magni/Modi, as I have heard some people prefer the originals).
I eventually chose the Beyers, however, because I found them generally more enjoyable with music, which may come as a surprise to some. While slightly smaller than the 701s, they still had a huge soundstage and imaging was better. I love how attacking they can be, when you let loose it's so easy to be caught up in a great song, while the 701s I found a touch analytical (some people may disagree with this assessment, just IMHO). I personally find the the 250ohm model works really well with the Aune T1, and can highly recommend that as a combo. The DT880s also beats the 701s (and just about everything else bar the HD800) for comfort, which is superb.
Either way, they are both brilliant headphones and I think you'll enjoy either. Just be careful with the 701 in particular, because with some amps it can be less pleasant to listen to. Just FYI, both headphones work really well with the Meier Audio Corda Rock, which is an excellent amp for the price. Good luck!