From the conclusion of the gemara on daf 3b, it sounds as though ideally one should talk using clean speech (lashon nekiya) but if it's shorter the other way then you should choose the shorter way of speaking (lashon ketzara). I think that two things in our gemara contradict this point:
1. We have two pesukim in the Torah quoted on 3a where the Torah chose the longer way of speaking because it was lashon nekiya.
2. The two students who were in front of their rebbe and one asked "lama moskin b'tuma" and the other said, "lama ein moskin b'tahara." The rebbe praised one of them. The simple way of understanding is that he praised the one who spoke b'lashon nekiya but shouldn't the other one have been praised because he used less words?
There are really tons of answers in the Rishonim on these questions (it's amazing how many Rishonim there are on Pesachim) but I'll just quote a couple for each question. The Chidushei haRan says that we need both pesukim to teach us different things. Rabeinu Dovid and the Baal Hameor answer that there are actually three levels. There is "clean" talking, "unclean" talking and then something in the middle. So you should go the shorter method unless it's unclean so in the pasuk quoted about the "mikreh layla" that would be unclean to say tamei so it uses the clean lashon there even though it's longer. However when talking about a woman riding, it's not unclean to say "rocheves" instead of "yosheves" but it's just nicer to use the latter. So if they're equal then we'll go with the cleaner language but we won't go out of our way for that.
To answer the second question the Ran says that the problem is that by asking why do we harvest b'tuma, it's actually unclear because it makes it sound like you must harvest b'tuma and that's obviously not true. Therefore, shorter is better but if it's going to blur the meaning then you should choose the one that is more clear. The Ran in the name of Tosafos (I don't see a Tosafos that says that) says the praise was really for the one who used the shorter language even though it was less clean he was right to speak in the shorter method.
The Maharsha answers both questions that if the cleaner way is longer then you have the choice which one to use. The reason the second student was praised was because he changed from the normal way of talking (similar to the gemara in Brachos 38a which says that R' Zeira wasn't impressed with R' Zvid because he said motzi instead of hamotzi - and motzi was obviously OK but he should've said it in the less obvious way). So the other student did nothing wrong but Rebbe (or Hillel) realized that the one who changed was destined for greatness.