ledwardj, welcome to the Forum,
Was out today in my 3.0si with a mate I often drive out with, Cherrym. He has a 2.5si and I exchanged Zeds with him today to see what he thought and so I could try his 2.5.
So, on absolute full bore my 3.0si will have the edge on acceleration but everywhere else the 2.5 was a very impressive drive. I really enjoyed it and it felt well planted and set up. Afterwards we talked about the Zeds and Mike wasn't full of regret at not having a 3 litre, he felt that with the sport button, his 2.5 was an effective Zed and I have to agree with him - I really enjoyed driving his Zed. It certainly keeps up when we're out on a spirited drive and that's the point - good drivers will exploit the capabilities of the Zed and you just cant be using all its performance, for most of the time, on public roads.
I have the 3.0si and I was impressed with Mike's 2.5si. (I've also had a 2.0i Sport and I loved that - had to keep it wound up but it was great fun on back roads in Northumberland - I also drive with Chezsherman who has a 2.0i Sport and he keeps up as well. . . then there's Geezah , an ex biker that gets the very best out of the 2.0 litre - further emphasising it's the driver as much as the Zed especially on the road).
I would buy on condition with the only reason to buy a 3.0si being it might be easier to sell in the future as they are quite sought after (and for that reason command more money). I guess it really depends on what you are like and what you want - if you enjoy using a car in a spirited (but safe and responsible) way on the road, either of the Zeds will offer that; if you want numbers, bhp and such matters are important to you, then only go for the 3.0si which will also be wonderful on the road. (The ///M is altogether a specialist Zed with associated high running costs - great if you can afford it).
Why have I gone for the 3 litre then? Well, having had a 2.0 litre I wanted to go for the best I could afford (£10500) in terms of performance and running costs and I was able to wait for and successfully find a BMW Authorised Used Car (AUC) at my price point. The 3 litre is not much more expensive to run than the 2.5 (it does use 4 or 5 mpg more fuel though) and when I test drove mine I was convinced within 200 yards. . . . it felt wonderful and I knew I would have a great time using it on the road - and my wife continues to agree with our decision.
Whatever you end up with you'll have a brilliant involving driver's car - do try getting out on a run with other owners - it will add another dimension to your ownership experience!
All the best with your search