Start a new project with the Devforce Winclient

Sorry, Erturk, I just don't understand the question.

DevForce WinClient is a newer generation version of DevForce which uses a newer generation of .NET.   DevForce Classic is built on .NET 2.0, which was supplanted first by .NET 3.0 and then by .NET 3.5. DevForce Classic is a very stable product, but we are doing no new development on it; in fact, most of our current development effort is focussed on a new version of DevForce based on .NET 4.0 (still in beta) and a major new version of the Entity Framework.

Under most circumstances, I don't think I would start a major new development project on technology as old as .NET 2.0. I might choose to do a small project based on that if I had lots of expertise in the older technology and the new project had a very limited scope and anticipated lifetime, and a tight deadline.  But the changes in .NET, especially the introduction of Entity Framework, have been quite revolutionary and at some point you probably need to bite the bullet and take on the new stuff. The good news is, there are some magnificent facilities in the newer versions of .NET (and the versions of DevForce built on top of them): especially, LINQ and the Entity Framework.

So, for most people and most new projects, I'd say .NET 3.5 and DevForce WinClient are much better choices than .NET 2.0 and DevForce Classic.

Having made that choice, you still have to choose between WinForms and WPF for a front end. We have some clients, including one doing a quite major development project, who have chosen DevForce WinClient with WinForms rather than WPF. But they started development (with a large team) more than a year ago and made their decision based on a tight deadline, lots of existing expertise in WinForms (and almost none in WPF), and auxiliary development facilities and tools which were relatively immature in WPF at that time. I doubt they would choose WinForms if starting their project today, as the support facilities and tools have gotten much better.

WPF, besides offering really cool facilities for building UIs, has the great advantage of working almost exactly like Silverlight; i.e., once you learn WPF, building a Silverlight front end is a very short step. And Silverlight is a great technology, offering the best of web (zero client deployment) and desktop (rich UI) applications in a single package.  So it's nice to know that by learning WPF you're putting yourself a step closer to that very attractive technology.