The latest ROI and revenue figures I have work out at: Espotting ROI roughly double that of Overture, but owing to the greater traffic Overture has generated approx 2.5 times the revenue putting it ahead in net profit.
For a low cost way of testing the water, I'd suggest going with espotting, and once you're happy with your campaign results, expand it to Overture to reach a wider audience.
It is interesting to note the different findings and would guess results will vary depending on your product. As Sugarkane suggests, you will need to test to find out what works best for you.
Preference currently for Espotting due to Yahoo connection, not really sure who Overture UK link with but whoever it is, it's not enough.
I had to deposit £50 with Overture UK (£25 with Espotting). At this rate it'll take 6 months+ to spend it all :)
From experience of running 2 campaigns for differnt types of sites, I'd agree with Crazy_Fool - you need to be on Espotting, Overture and Google Adwords.
Espotting have delivered more visitors for my campaigns for a lower cost. Their UK partner list at the moment states:
Yahoo, Lycos and HotBot, Ask Jeeves, UK Plus, LookSmart, Netscape & InfoSpace.
They also seem to be partnering with destination sites too, like TV's Channel4.com and on some areas of Kelkoo (I noticed today they power ISPs there).
Overture mainly appear on MSN, AltaVista, AOL and FreeServe.
So I'd suggest you start with Espotting. They let you take any relevant search words, not just those with a certain volume of traffic, unlike Overture. This can help you test your campaigns before commiting a bigger spend.
You have to decide if you need critical mass or volume.
For example if you are offering a newsletter sign-up because you want to build a large opt-in e-mail list then Overture US is likely to deliver more visitors for you.
You need to look at your specific model, b2b, b2c. You need to look at the geography of your target audience (UK, US, Europe, global) and construct your campaign accordingly.
You also need to use the tools provided as though they are gold pans. So if you are running an overture campaign you should categorise everthing under say 20 cents (20p) and manage those from one account, if there are dramatic fluctuations it won't cost you lots, but if you have bids of several £ or $ per click and you pay 20-50% more (or 20-50% less if you are good) than you need to on a few hundred clicks a day it mounts up.
I know some people use software to manage this stuff but you can't get tactical with the software (in my opinion), you either bid up or down, you can't increase for prime time, decrease at lunch time (assuming you are running a b2b campaign).
It's horses for course, they all have their place, will deliver good results, but can be poorly managed which doesn't make them bad providers.