I'm coming to this discussion late, but here are my two cents.
First off, the modern "fetchlands," i.e. Arid Mesa, are easily much better than gates and the less sought after "fetches" like Evolving Wilds and Terramorphic Expanse. They let you get shocks, they get dual colors, lots of stuff. They're staples in modern and legacy decks too - hence their hefty pricetags.
So if we exclude them for a moment, the focus seems to be on the comparison between playing gates and the lesser fetches (Wilds/Expanse). To this end, they're both not bad options especially in a standard format that's about to lose all the M13 and Innistrad duals.
Why go Gate?
Gates offer the benefit of a dual color land. If you're running something with a LOT of color specific casting costs, well, this wins hands down. Yes, it comes in tapped just like the basic from the baby fetches, but which would you rather have? A tapped basic or a tapped dual? The key here is the flexibility - which comes at the cost of some speed. (Nevermind the shenanigans if you start playing Maze's End or some of the gatekeepers).
Why go Baby Fetches?
These are often best suited if you're running a deck that really just has to worry about splashes of other colors. Much like the group has pointed out, Farseek has been huge over the past year as it would let people go grab the shocks they need for all their color fixing. Admittedly the baby fetches cannot go grab a shock for you, but if you only need ONE white symbol in a deck, you can easily drop in a single plains, some shocks that have white, and perhaps 2-3 baby fetches with the knowledge that you will be able to get that white symbol rather reliably. BUT, perhaps more importantly, the baby shocks come with the added advantage of helping to thin out the remaining cards in your deck. It might not seem like much, but if you have 50 some odd cards remaining in your library, removing that one more land will increase your odds of drawing that one card you're looking for, improving deck flow and consistency.
So which is better? Well, to be honest, it's entirely situational. If you're running two colors (or more) you should run the gates as they're essentially duals, just a bit slower than we've admittedly been spoiled having. If you're just splashing a color, the baby fetches will work in your favor in a few ways.
Then again, Theros might give us reprints of the original fetches for all we know...