how much is this MacBook Pro worth & Need advice on which MCP to buy

Tell him the current "brother" price is $50.00.

Check what they are going for on eBay and deduct a few bucks for the imperfections. Just type that model in eBays search engine.

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The market values of all used computers are determined on Ebay nowadays . Check Ebay's recent completed sales of the computer model in question to get an idea of what the one you're buying is worth.

You say you're going to buy your brother's MBP, but then you go on to say you're going to ask for advice on which model to buy. Does he have more than one to offer you?

Assuming that most completed sales of your brother's model on Ebay probably happened before the new models were released, the used value of the older machine will have diminished a little since then: probably on the order of $100-$150. So you have a choice between a 15" machine with a few months left of its one-year warranty or, for an extra $150-$200, a 13" machine with a full year's warranty, a marginally slower processor, a bigger hard drive(?), more RAM, better battery life, and perhaps more advanced graphics. I wouldn't call the newer machine a slam dunk winner in the comparison, but on balance it's probably a better deal, if you can stand the tiny screen (I couldn't).

I recommend buying AppleCare, no matter which machine you choose.

On second thought, let me revise my AppleCare recommendation. If the dings on the old machine are anything more than purely superficial, don't buy AppleCare for it. The likelihood is that if anything goes wrong with it, Apple will blame the problem on whatever accident caused significant scratches or dents in the case, and AppleCare won't cover the repair. The same thing may be true of the original one-year warranty. So if there are any marks on the case indicative of anything more than occasional gentle jostling or a brush with a metal bracelet, that machine's warranty may already be void. This makes a new machine look considerably better in comparison, though the little screen is a real drawback in my opinion.