Is it better to pick up a Chapel or Steward?
I would (almost) always prefer Chapel.

Honestly, if Chapel's on the board, I will buy it before my first shuffle, almost no matter what. This is because I love putting together combos, and Chapel is the fastest way to thin a deck, which then removes most of the chance from getting a combo to go off.

@ColinD's 5/2 hypotheticals all sound playable. (Steward/Fool's Gold sounds especially fun to me, especially if you can get another cheap card with +Buy.) But in every one of those cases, I think Chapel would work better than Steward. For example, in the Fool's Gold case, you can literally trash everything but Fool's Gold (and the Chapel) very quickly. If you can pick up a Market or two (or anything with +Buy) you'll soon buy out the Fool's Golds -- and that's the point when you want card draw. After the Fool's Gold pile is empty is the appropriate time to pick up an action for card-draw, though hopefully there's something better than Steward's +2 Cards.

The thing about Chapel is that it makes your deck fast. If you open Chapel/Silver and start trashing like crazy (and if you open with a Chapel you must buy something else that produces coin or gains cards), pretty soon you'll be down to a 7 or 8-card deck with no chaff. The good cards you've bought come up again really quick. It's fine if your non-Chapeling opponent has 2 Golds and you only have one if you're playing yours almost every turn! The situation is even better for terminal actions: a single Goons in an 8-card deck is much better than 2 Goons in a 16-card deck because it'll come up just as frequently and there's absolutely no risk of collision. This is why Chapel is so good at setting up engines: the engine cards are about the only ones in your deck, so they come up together every time.

Decide up front if you're going to use Chapel; if you are, buy it before the first shuffle. Waiting for an unlucky $2 hand defeats the purpose of Chapel: speed. With Chapel, you want to trash all the Coppers and Estates ASAP, and if you've waited a few turns and already bought several other cards you want to hang on to, you've already missed the boat on the Chapel.



When not to use Chapel:

Don't use Chapel for a Big Money. Big Money + Chapel is slightly slower than just Big Money. 

Chapel is great because it can trash 4 cards at a time. Chapel is not great when your hand is small. If you've discarded down to 3 cards, your Chapel is now a crappy Steward, lacking the options that round Steward out into a useful card. Chapel is fast enough that expensive discard attacks (especially Goons ($6), but also Ghost Ship ($5) and Margrave ($5)) aren't too concerning. (Torturer even less so, just take the curse and immediately trash it.) But Militia and even Cutpurse can get in the way. This is especially true in > 2-player games where you have more than one opponent causing you to discard. If you almost always have a hand with &lt; 5 cards, Steward is probably a better choice than Chapel.

Jack-of-All-Trades can also compete with Chapel. Chapel will be much more effective at setting up an engine, but it needs to be a darn good engine to beat "doublejack" (Big Money + 2 Jack-of-All-Trades).

@User1873 astutely pointed out that you need to consider available card-stealing/trashing attacks in evaluating Chapel. Thief, Noble Brigand, Pirate Ship, etc. steal/trash treasures and make a Chapel strategy riskier. A lucky attack or two could leave you in the unfortunate situation of needing to buy Coppers to afford a Silver! Any heavy trashing increases your treasure density so much as to make treasure-stealing cards very appealing. Masquerade can also make you give up good cards if they're the only ones left in your deck. Like the drawback of discarding attacks, the risks are exacerbated if several opponents are using stealers.