depends on what you're planning to do with it - i guess you're talking about 13" models, right? from a speed perspective, it looks like the newer i5 is actually the by 50% faster processor. but 16gb of ram and 500gb of ssd is also pretty nice to have. if you're going to do stuff that needs more ram (video/photo editing), there will be more swaps to the probably faster but smaller 256 gb ssd. 256 is usually a pretty tight fit, i constantly struggle to keep 10-20 gb free on my old 2009 mbp with 3rd pary ssd - and i don't really do anything with that machine any more. but that also really depends on your type of computer usage. the old one got a disc drive if you need that (which might or might not bite the bullet soon, but can be replaced, even with a second ssd) the battery of the old one will be older and didn't even last as long as the new one when it was new. but you can replace the battery by yourself, while you have to send the new one to apple for that.
still, personally i'd go with the newer one because of it's faster (longer lasting) processor, newer battery, smaller form factor, retina display and because it's cheaper. you're probably not going to use it as a video editing machine anyway (or you'd be thinking about the 15" pro with the discreet graphics), and for office work, surfing the internet and some light multimedia-work it should be fine for years to come.