Inventors' Fair gives artifact-heavy decks free life every turn if you control 3 or more artifacts, and that's not nothing. The Fair also can act as a tutor for any artifact card you want from your deck. While it has to be sacrificed to do so, it's still a strong ability.
Whir of Invention is an artifact tutor that puts the card directly into play, and can even have its casting cost reduced by tapping artifacts with Improvise! Both of these cards are really solid investments as what they do is just really good. I don't really like Whir foils at the moment, as their price is trending in the wrong direction. But I do like Inventors' Fair foils, if you want to go deep on them. I just don't see either of them being reprinted in foil any time soon.
Inventors Fair and Whir of Invention both seem like pretty safe investments, even if you don't plan on building the decks anytime soon. To me, the Fair is the better investment as it's more versatile being colorless. Whir has three required Blue mana in its mana cost which does restrict its flexibility a bit, even though many artifact decks are primarily Blue anyway. But if you're looking to break into investing in Modern Magic the Gathering cards, these are two strong candidates for adding to your Magic collection.