I think you are attacking it from the wrong angle by trying to encode all posted data.
Note that a "<" could also come from other outside sources, like a database field, a configuration, a file, a feed and so on.
Furthermore, "<" is not inherently dangerous. It's only dangerous in a specific context: when writing strings that haven't been encoded to HTML output (because of XSS).
In other contexts different sub-strings are dangerous, for example, if you write an user-provided URL into a link, the sub-string "javascript:" may be dangerous. The single quote character on the other hand is dangerous when interpolating strings in SQL queries, but perfectly safe if it is a part of a name submitted from a form or read from a database field.
The bottom line is: you can't filter random input for dangerous characters, because any character may be dangerous under the right circumstances. You should encode at the point where some specific characters may become dangerous because they cross into a different sub-language where they have special meaning. When you write a string to HTML, you should encode characters that have special meaning in HTML, using Server.HtmlEncode. If you pass a string to a dynamic SQL statement, you should encode different characters (or better, let the framework do it for you by using prepared statements or the like)..
When you are sure you HTML-encode everywhere you pass strings to HTML, then set ValidateRequest="false" in the <%@ Page ... %> directive in your .aspx file(s).
In .NET 4 you may need to do a little more. Sometimes it's necessary to also add <httpRuntime requestValidationMode="2.0" /> to web.config (reference).
[HandleError]
When you provide only the HandleError attribute to your class (or to your action method for that matter), then when an unhandled exception occurs MVC will look for a corresponding View named "Error" first in the Controller's View folder. If it can't find it there then it will proceed to look in the Shared View folder (which should have an Error.aspx file in it by default)
[HandleError(ExceptionType = typeof(SqlException), View = "DatabaseError")]
[HandleError(ExceptionType = typeof(NullReferenceException), View = "LameErrorHandling")]
You can also stack up additional attributes with specific information about the type of exception you are looking for. At that point, you can direct the Error to a specific view other than the default "Error" view.
For more information, take a look at Scott Guthrie's blog post about it.
Best Answer
You should think of WCSF as guidance about how to use the existing WebForms infrastructure, especially introducing Model-View-Presenter to help enforce separation of concerns. It also increases the testability of the resulting code.
If you can target 3.5 SP1, you can use the new Routing system with a traditional WebForms site. Routing is not limited to MVC. For example, take a look at Dynamic Data (which also ships in 3.5 SP1).
This is true because it uses the new abstractions classes for HttpContext, HttpRequest, HttpResponse, etc. There's nothing inherently more testable about the MVC pattern than the MVP pattern. They're both instances of "Separated Presentation", and both increase testability.
In Model-View-Presenter, since the outside world interacts with views (i.e., the URL points to the view), the views will naturally be responding to these events. They should be as simple as possible, either by calling the presenter or by offering events that the presenter can subscribe to.
Model-View-Controller overcomes this limitation by having the outside world interact with controllers. This means your views can be a lot "dumber" about non-presentation things.
As for which you should use, I think the answer comes down to which one best suits your project goals. Sometimes WebForms and the rich third party control vendor availability will be preferable, and in some cases, raw simplicity and fine-grained HTML control will favor MVC.