I love the '57 Classics if they come in a guitar, but too many options in the aftermarket to buy them as replacements. Gibson wants a ridiculous $200 each for the 4-conductor type of '57 Classics...
I like to swap pickups in humbucker equipped guitars... the '57 Classics get their tone from the Alnico 2 magnets... Burstbuckers have Alnico 5 magnets... there's your tone difference... Lindy Fralin has a really cool page that discusses the different types of Alnico magnets...
Alnico magnets are widely used as guitar pickup magnets. They come in different grades and strengths - what does this mean for your guitar's tone?
www.fralinpickups.com
As far as if I were swapping pickups and wanted some nice Alnico 2 magnets in them like the '57 Classics have, I'm looking hard and long at the Seymour Duncan Pearly Gates pups. 4-conductor (I love to coil split my hummers), Alnico 2 magnets...
HALF the price of the Gibson 4-cond '57 Classics (the Gibson folks smoke too much crack when designating prices on their products... like (cough) $4,699.00 for a Les Paul Custom... WTF ???)
My next test project for the SD Pearly Gates pups is going to be putting them in re-wire of an Epiphone Dot (ES-335) (I will also totally re-wire it with 50s/vintage wiring, new pots, caps, wire, jack and the neck/bridge Pearly Gates set.) That's kind of a standard hobby thing I do that I've had excellent results with on other guitars. I'll also replace the top hats with speed knobs because I will adjust the nut on the pot so its easy to get my fingers under for push/pull coil split.
Here's the poop on the SD Pearly Gates
The Pearly Gates vintage output passive humbucker pickups pack ton of focused midrange attitude with raw Texas punch and sizzle.
www.seymourduncan.com
As far as choosing a new guitar, well first I'd let some other clod take the 30% hit out the door on the new price, and look for used mint. As soon as you walk out the door with any brand new guitar, you instantly lose 30% or so of the value. If you buy used-mint, the guitar holds all of the value after the purchase (unless you pay too much for it in the first place)
And... since you are wanting to add another guitar, and with Gibson on the head stock, I wouldn't waste money swapping pups on a Gibson. It should either have the sound you want, or it doesn't come home. So I recommend an in-store test drive. See what you like, then shop the Internet for the model you want used-mint.
As far as pickup swapping, I find excellent deals on Epiphone guitars, then do a total re-wire from pickups to jack. I have some really nice projects that I've made that way.
I have a dumb ass cousin that has more money than brains who bought a brand new R9 Les Paul for the really stupid money Gibson asks for these these days..., then swapped the pickups... then sold it wholesale to a vendor that sells used/mint on the Internet. "because he had 3 x R9's and didn't need this one"... ugh... so IMHO, he played mad scientist with a super expensive guitar, f-ed it up real good with pickups it was not engineered to go with, plus it forever will never be "all original"... duhhhh...... then like a spoiled brat with too many toys, sold it for pennies on the dollar wholesale, hahahaha
Don't be that guy...