generally they do better, until recently you could send the ad alot of times direct to the affiliate page, but now they are implementing a quality score system so i wouldn't do that. but if you focus on google and get a good ad up there, the volume you get will just kill anything msn can offer.
MSN AdCenter conversions for our retail e-commerce clients are high, search traffic is low AND cost per conversion is very high! Consequently we only run MSN AdCenter campaigns during the 4th quarter.
MSN sends me more traffic than google. Google only sends me 5 visitors per-month for a $0.30 keyword bid but msn sends me 100 visitors for same keyword but at $0.20 bid
I took advantage of a special offer they had and got $100 in free clicks. I set up a campaign exactly like one that had been working fairly well with Google and got 0 conversions. Others may have had different experiences, but for me I will avoid AdCenter
I took advantage of a free coupon too. Then they "accidentaly" overcharged me and took the balance out of my account... they are not easy to work with, i hate to say it but google seems better to me.
msn adcenter has a good conversion rate I've found but you need a lot of quality keywords in order to get a decent amount of traffic and clicks to make a profit