Assuming your final deck should only have 1 silver (almost any engine), then it is probably better to get the silver on the non-chapel T3/T4 turn. This is more important when the cantrip is a $4 to avoid having two silvers in your deck by T5, with a chapel deck. The silvers are kind of annoying when you are trying to chapel. They often just get in the way of early trashing (cantrips won't).
Really? I play a lot of engines and I'd say very few are that silver intolerant. For instance, most engines depend heavily on getting key 5 coin parts (minions, margraves, torturers, libraries, festivals, etc.) Take a lib/fest/chap setup, I need to consistently hit 5 to get my components, until I get my second festival, a second silver is not that bad. Yeah, I'll likely trash it (and the first silver) so I can have a leaner deck, but really I'd rather have better odds a lot of the time at hitting the power 5s than having a leaner deck struggling to get to 5. If there isn't a better way to get components, two and three silver are not out of the question.
Now if there are terminal silvers out, sure I'll buy one of those in hopes of getting a 5 on T3/4 or buy a second silver then, but if the choice is between a weak cantrip and a silver, I think I'll take the silver. Afterall it is an odds off bet that it will collide with the chapel on the first shuffle, and decent odds of getting the 5 coin I need to get the engine humming.
I'd be inclined to go cantrip/chap IFF the cantrip provided a long term benefit. WV with peddler out is a perfect example; the odds of bad shuffle luck go down AND I have a better chance at picking up payload for the rest of the game. Walled village, well it might give me better shuffle luck, but if I'm going engine with that, then I need to have some very strong payout coming and that normally means more expensive cards so I want some economy rather than ditching all my coppers and having to slow build up.
Another big factor in the silver vs cantrip issue is the power of any 2's out on the board. Yeah silver is dead if it collides with the chapel, on the other hand, something like haven, xroads, hamlet, courtyard, pawn or native village means that I can buy an engine component during the collision. If I want to buy a few 2 coin cards, I may as well get them early.