I'm toying with adding a new lens to my collection for weddings, location portrait and nature/wildlife. I've currently got a 17-40 L and 24-105 L as my main two lenses, used on a 5D Mk II, and a 40D as backup.

The 70-200 f/2.8 L IS II would be ideal, but it's way too expensive and heavy, so it's not practical.

My main choices are between the 70-200 f/4 L IS (I love the IS in my 24-105) or the wide and heavier, but non-IS 70-200 f/2.8 L

They are similar prices, and both very good reviews. For me the f/4 is smaller and lighter, therefore I'll likely use it more and the IS will be great, however the f/2.8 is bound to give better portraits, bokeh and for weddings (which make up a chunk of my work) and I'm thinking the lack of IS will be made up by the faster aperture. I've not had the luxury of these faster lenses, mainly due to weight and expense, so any thoughts from users?

The other option is just to go for something like the 85mm f/1.8 and although I suspect I'd get great portraits from this, at a wedding, it's far less flexible than a zoom.

Have you considered renting each lens for a bit to see how they fit with your style?

It's not something I've done yet, but when large amounts of cash are in play, it's worth testing before spending. All the podcasts recommend it.

Yeah I have considered it... tempting and I nearly did for a wedding a did last month from these people: http://www.lensesforhire.co.uk
but decided to save the cash to actually buy. It's probably a good idea, but if I hired two to compare that's £150 to £200 gone, which is a quarter of the lens price!

Have you hired any?

No, but I know it's recommended as a way to test a potential purchase with your kit.

Like you, I prefer to buy what I need. If I suddenly found a requirement for a 800mm telephoto, I would hire it!

Better than the 10K purchase price of the Canon 800mm!