TBH, “prestige” brands are not the best for space. A couple of years ago I was up in Germany with two hire cars - a 520d and a Focus - for 7 people and luggage. We ended up with three in the BMW and four in the Ford, because it had loads more space.
Skoda Superb. Bloody huge inside, masses of legroom. A standard S or 7 wouldn’t get anywhere near it for legroom. Would be interesting to compare it for space with the L versions of each. My father-in-law had one and apart from an issue with the leccy handbrake not releasing, was very happy with it. Get the auto rather than manual to avoid this issue. And the estate, with the high output diesel would make a great load carrier also.
I think the current and previous generations of the 5 & 7 series share a surprising amount of mechanical parts so can’t see why a 7 should be any worse than a 5.
I have a 530D which gives 40MPG so a 730D long wheel base shouldn’t be too bad on fuel.
EDIT: I meant that both series of the same generation share parts not intergenerational sharing of parts.
Using my advanced car knowledge, I can tell you that one is a saloon and one is a hatchback. Were it not “prestige”, perhaps it could transform into a hatchback at will.
Notwithstanding that factoid, the BMW has 150 litres more boot space.