I will add my 2 cents on costs - In a most of my implementations, getting SiteEdit 2009 installed and tested typically has taken less than half a day per environment. Make sure you apply all the hot fixes from SDL Tridion World to get it to work with the latest browsers.
The new UI can optionally use something called 'Session Preview' (to enable fast publishing) which makes use of several Content Delivery technologies such as OData. If you are not already using these in your implementation, then their is likely to be a considerable investment into infrastructure/application design to get it designed/installed/working/tested (I have heard cases where this has taken over a month), which will make the Experience Manager considerably more expensive to implement in the short term. If you don't use the 'Session Preview' feature, (as Frank has already said) the implementation time/cost is similar, but you will not benefit from the new fast publishing features of the newer product.
As for functionality - the two environments look and feel very different. Experience Manager is clearly the direction the products are moving towards and provides a much slicker interface. So if your client is new to SDL Tridion, I would suggest using it, however if they are a long time SDL Tridion customer who has experience with SiteEdit 1.3 or 2009, and you don't plan to take advantage of some of the newer features in the short term, I would be tempted to stay with SE 2009, and make the shift to Experience Manager when they upgrade to SDL Tridion 2013.