Although the two cases can easily be differentiated in many ways, nevertheless, on the surface , there is an interesting paradox in the opinions of Beith Shamai and Beith Hillel.
Beith Shamai, who earlier in the Tractate (16a) state "from the first day of the week keep in mind [in terms of food assignment] the Shabbath", are lenient concerning kviuth due to Shabbath, whereas Beith Hillel who state (ibid.) "Baruch Hash-m Yom Yom", are stringent concerning kviuth for Ma'asros due to Shabbath.
be'Kavod u'Bracha,
Mordechai Schwimmer
That's a very interesting point. To me it would seem that to the contrary: that is the very reason for Beis Shamai's ruling regarding Terumos. When Beis Shamai designated food for Shabbos, he knew that there was a good chance it would not be used for Shabbos (since he might find a better piece). Beis Hillel, on the other hand, used whatever was designated for Shabbos only for that purpose. Therefore, designating for Shabbos made it "Shabbosdik" in his eyes.
Thanks,
Mordecai Kornfeld
Kollel Iyun Hadaf