If we're comparing Junk Dealer and Sentry, I'd agree that purely as a trasher, Junk Dealer is stronger than Sentry. Unless you can get Sentry before your first shuffle, it becomes very difficult to line up your junk within just the two card range Sentry looks at. Additionally, Junk Dealer's ability to trash from your hand usually gives more flexibility and potential targets to trash than Sentry does. I've had multiple games where I play Sentry with a Copper or Estate in hand only to reveal two action cards (which I put back, hooray)--if I'd had a Junk Dealer instead, I could have at least trashed something from my hand and had +$1. Yes, when you do flip two junk cards with Sentry and trash them, it's amazing, but that's remarkably rare--if you don't buy Sentry until after your first shuffle, it might happen once in a game if you're lucky.
As a trasher, I think I'd only choose Sentry over Junk Dealer on a 5-2 split, as the chances of trashing two cards with it are highest if you can get it right away. Otherwise, if I can't hit $5 until turn 3 or 4, I'd go with Junk Dealer for a more reliable trasher that can provide some economy.
That said, once you're into the mid to late game, I do think Sentry becomes much stronger than Junk Dealer as it doesn't force you to trash, which makes JD a liability sometimes. Instead, Sentry acts as a half-Cartographer, filtering your green cards from the top of your deck, while leaving the actions you want on top, and perhaps trashing the occasional Curse that pops up. True, Sentry won't be the star of the show late game, but it also won't be a dead card in a hand full of provinces and actions like JD can become.