We got my engagement ring from Diamond Geezer. The name doesn't inspire confidence but it was brilliant. I went to hatton gardens first and didn't enjoy it at all - I wanted an emerald cut diamond and there were mostly brilliant cut solitaires on show. Most of the jewellers have a few loose stones and then print-outs of the details of thousands more diamonds which they can order. But you can't actually see them all anyway and I felt rushed to consider the trade-offs between the 4 C's. so I ended up doing online research on stones and then we took the plunge. Diamonds are a commodity anyway and as long as they are properly certified by one of the reputable authorities (can't remember the names now but there are two main ones iirc) then you should get what you expect.
I am not sure whether Diamond Geezers has exactly the setting that you would want but they have a lot to choise from. They also have details of thousands of stones - they don't stock them but catalogue available stones from a lot of suppliers, and can then get the one you choose (so they don't have the cost of carrying stock, iyswim). You can take your time putting in various parameters to trade off, say, cut and colour, and see what it does to your bling. On the DG website you can 'see' what the ring will look like.
I think it took about 3-4 weeks to arrive but was actually a bit ahead of the schedule. The ring itself was immaculate. I had a platinum setting with a central emerald cut diamond with two small baguettes on the shoulder. The setting was perfectly done (no wonky claws, heaven forbid). I can't remember how it was packaged because I ripped it off so quickly! You get the original certificate for the stone with it so that is your assurance of what you have bought (you could also get it valued independently but I haven't yet).
Basically though I felt they had lots of choice, a great website so you can search a lot of diamonds and get a view of what the ring would look like (and that was pretty accurate, although you can't get the full sparkle effect on screen!). In spite of my reservations about buying online it was very good experience and much more for our money than the high street. Even if you buy elsewhere I'd recommend the website for a browse and to use the viewer.
As for the trade-off, I think the importance of clarity vs colour depends on the cut. Emerald cuts show inclusions more so I went up on clarity but still went for good colour rather than a huge yellows stone.
Hope that helps, and at least gives you the confidence to go for it. I am delighted with mine!