Both electronics, especially nav and such, are out of date. So it comes down to which drives best and is most fun. For me, that is the e53. The e70 has enough polish to deaden the fun a bit The e53 is a pretty raw animal, a beast. And with Dinan...no contest. Just get a Garmin or upgrade the nav system to Dynavin and add a camera, and you are current with the X's of today (other than the M, which seems like it could be fun).
On the e53 there are well documented, known issues or annoyances. My air suspension has been fine, but if it starts to fail I'd replace it with conventional stuff. So that is no barrier. The e70 also has its issues, many are the same. So since I prefer the feel and drive of the e53 I would go that way all day long (in fact, if I found a really good black/black e53 4.8is I would sell the e70 and keep two e53s). You may prefer the opposite, but you asked my opinion
