You attach the useraction in the URL you are redirecting to.. eg:
https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=<TOKEN>&useraction=commit
The two PaymentTypes you have listed are the same according to the API.
Also, based on this sample code you should be able to add the useraction=commit
with the MerchantSDK.
Line 420 shows:
CurrContext.Items.Add("Response_redirectURL", ConfigurationManager.AppSettings["PAYPAL_REDIRECT_URL"].ToString()
+ "_express-checkout&token=" + setECResponse.Token);
and can be changed to:
CurrContext.Items.Add("Response_redirectURL", ConfigurationManager.AppSettings["PAYPAL_REDIRECT_URL"].ToString()
+ "_express-checkout&useraction=commit&token=" + setECResponse.Token);