1) I think that according to the poshut pshat it is really the same thing. Rashi in the Mishnah writes that the Torah does not want the people returning from the war to be embarassed. The reason they would be embarased is because of the onlookers. If nobody would see them going home there would be no emabarassment involved, which is why Rashi in Chumash mentions the onlookers.

2) However I can say, more in the way of drash, a vort based on something I found in sefer Or Yesharim by Rav Moshe Chaim Kleineman zt'l; in Eshel Avraham, by Rav Avraham zt'l of Slonim. On page 284 #25 he writes:-

"One should rememeber that it is better to be embarassed; even several times; rather than embarassing somebody else; even once; even if one does this LeSheim Shamayim".

Now we can say that the aim of Rashi on Chumash is to highlight the Mussar that the Torah teaches us. That is why Rashi stressed that the Torah tells soldiers to return because of their new house or vineyard, so that the onlookers should not stumble and embarass those going home and mock them for their Aveiros. It is worse that the onlookers embarass, than the returners being embarassed. However Rashi's purpose in his commentary on Shas is somewhat different. He wants to make sure that we get the correct pshat in the Gemara. The Mishnah is discussing the soldiers returning home, not the onlookers. That is why Rashi writes that the soldier going home, should not be ashamed.

3) There is a source in Shas for the idea written above by Or Yesharim. This is from Tosfos Bava Kama 23a DH v'LeChayev, who writes that a person must be more careful not to damage others, than he is careful not to be damaged himself. Rav Avraham of Slonim extended this idea to apply not only to damage but also to embarassment.

KOL TUV

Dovid Bloom