I owned an E60 (replaced with a F10) and have an E70 (NON-Diesel) - comparatively there was no major difference in repairs / parts and if you do your own maintenance it will be similar as well. I found that issues were quite “Predictable” eg there are a number of well known and discussed weaknesses in these models (eg cooling system, front suspension, etc). I believe the e61 has an air suspension which you may find in some versions of the e70.

The interior space and easiness to get in/out are definitely a plus in the e70 and is the preferred vehicle for road trips / errands, however, the fuel economy is not similar and for daily commute the e60 (and now the F10) where the obvious choice for a wide margin - expect a 25-30% increase compared with your e61 (diesel maybe less)

Second row space in our e70 is significantly better compared to the e60/F10, maybe because we have third row seating and the second row can be rolled further back and reclines a bit.

Tires...my e60 was RWD and ate rear tires quite rapidly, specially on the insides due the rear camber and because it had staggered set; I think I went through tires faster on that car vs the e70 (note, I drove it more aggressively than the e70)

Just my 5 cents.

__________________
E70 - 3.0si & F10 - 535ix
Previous Cars:
E34 - 525i, E39 - 530i, E60 - 530i