You should use the actual url for the canonical url, you do not want the canonical url to be an url that does a redirect. You want to avoid un-needed redirects as much as possible. When there is a redirect the response is sent to the browser with status code 302 (object moved) for a temporary redirect or 301 (object moved permanently) for a permanent redirect and the new url is provided then the browser navigates to the new url. So you have extra web requests going on for every redirect. For good SEO avoid un-needed redirects.
You certainly don't want your canonical url to do a permanent redirect. Its like saying this is the correct url but then saying no it isn't this page moved permanently to this other url. Don't make your page use extensionless urls and then have .aspx in the canonical, its a very bad idea.
There has not been any change in how urls work in mojoPortal.
mojoPortal only creates a 301 when you change the url of an existing page so that the old url will redirect to the new one. It does not create any redirects for new pages and you should not need any. You either use .aspx or you use extensionless urls but not both for the same page.
[having canonical url with .aspx, in case if the site needs to b revoked to "Mandatory Extension" in mere future, if required]
The only way that would happen is if you think you may need to move the site to an older version of IIS where the extensionless urls might not work like Windows 2003/IIS 6. If you really think that is a risk to be concerned about then my advice is don't use extensionless urls, just stick with .aspx and don't change them. You definitely should not have the canonical url with .aspx and the actual url without it.
Hope that helps,