You said that people you will likely play with will generally not have the same level of experience as you - personally in that situation I would avoid getting Intrigue. There are many cards in that set that I would imagine new players probably won't enjoy (although like every set there are some good ones too). In particular though, in Intrigue there are many cards where you have choices of which benefit to receive, and at the end of your turn when you are counting your available treasure for instance, it might not be easy for a new player to keep track.
I would say that Seaside would be the best expansion to get for playing with new players, there are many cards that are simpler to keep track of or just fun in there, but that's just my opinion. It was, until buying Dark Ages today, the only expansion I owned and wanted, so I am maybe a bit biased, but I have had success playing it with new players.
Ultimately though I would advise getting the basic (Dominion) set because sure, it may be a bit redundant since others are likely to have it too, but when you introduce new people to the game using only your own cards, it's always great to have Dominion available. If you were to go for Dominion, you could always then pair it with either of the smaller sets, Cornucopia or Alchemy, saving a bit of money and still making sure you have sets that other Dominion players you meet are perhaps less likely to own; with Dominion around you have 25 kingdom cards to augment a smaller set and wouldn't have such a problem of too little variety that you might have with just Alchemy and the Base cards.
Oh and regarding the question of whether Cornucopia is a small set - you can check out all the sets in this very site (http://dominionstrategy.com/card-lists/ and Dark Ages has its own recent post but is not yet in this list)
I'm not sure I'm going to be playing with new players, just (probably) not ones who've been playing on iso almost every day for the last month. Anyway, I think Intrigue makes the most sense economically because it's only ~$15 more than the "base cards" set, and I need those either way. It's basically 25 kingdom cards for $15. What a steal! I might get another set as well.
As for Seaside, I think it's one of my favorites as well, but the fact that my sister already owns it makes it seem like sort of a silly choice. I also agree that learning on Base is probably best, but it'll be around if I need to teach newbies.
I actually had already found the "All Cards" article, I just forgot that Cornucopia was small because I've never played it IRL, just on Iso.
Thank you all for your help and advice!
Good luck with your purchase, but for what it is worth, I will say that Intrigue is my least favorite set. And, here is why. Saboteur--weak and swingy card. Torturer--setting up a chain of these will likely mean you won't have any friends on campus (note: I don't mind Torturer chains), Wishing Well (requires too much memorization of one's deck), Minion--not a horrible card, but an annoying attack, Great Halls-generally a worthless card but it does give one VP, Conspirator--a fun card, but I don't think aside from Wishing Well and Great Hall and Mining Village, I don't think there is much chance for this to shine Shanty Town--probably the worst village, not counting Ruined Village or Necropolis, and hey, you start the game with Necropolis in DA games Mining Village--don't get me wrong, this is a great card, but it is not very newbie friendly because no one wants to sacrifice it. This is probably my second favorite village, but it took me a while to figure out how to play it. Secret Chamber is also in this set, and that card took me forever to wrap my head around, it just slows games down a lot, especially irl. I will note that I am liking SC more these days, and I think it is underestimated a lot of times.
Oh, and Scout. No explanation needed
But, there are some highlights. Steward is one. Swindler, if you don't mind the attack. And, if you do like attacks as mentioned above there is Torturer and Minion. Did I mention that this set is very attack heavy. There are Nobles and Harems which I don't have an opinion on one way or another, but some people like them. Masquerade is in there. Duke is in there, but every set has an alternate VP card. There is also Bridge which is a fun card. Tribute is also in the set which has nice synergy with all the dual type cards.
I'm sure there are some other good cards in the set, but those are just my two cents. And, really, I don't think newer players will be fond of the heaviness of the attacks too much. I think Intrigue is the worst set when it comes to attacks.