If you haven't made it too far into OWA land, then let me forewarn you about some things. There are two version of Excel Services. The one that comes with SharePoint and the one that comes with OWA. The truth is:
You can have one and only one! – (update: I should have added here, "in certain areas" to get the point across)
UPDATE: Evidentally, some think it ok and acceptable, to view excel reports through the excel services web part (with the limited viewing area) rather than the more straight forward way of just viewing it directly in the browser. I'm not one of those people.
You must decide which one you want. In our upcoming book, I point out the various difference between the two versions. The net net is if you want advanced Business intelligence features, you must use Excel Services. If you want advanced viewing and editing, you must use OWA. You cannot target web apps, content databases or site collections for the decision. It is a global farm setting decision.
I find this unacceptable to the community as a whole and I can see a service pack coming that enables us to configure this at least on a web application level and if possible on a site collection basis too.
That being said, it is true, as pointed out by Bryan Hart on my Facebook page, that you can disable the "Viewing" action to allow Excel Services to view the files, but allow editing to be done by OWA. This is done with the New-SPWOPISupressionSetting cmdlet.
At some point, this will all change with new features and versions and hopefully work much better than it does now.