Slightly faster CPU and GPU. Force Touch touchpad. Faster SSD.
I would have to disagree that the faster SSD make much of a difference outside of certain specific applications.
Mostly +1.
If you compare models with dGPU, dGPU is a little better than "slightly" but still meh.
Overall IMHO only the Force Touch trackpad is a noticeable difference. (And for some it's even considered worse than the old mechanical one.)
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Both are stock base models, so Apple tried to fix my MacBook 4 times and after every repair something else appears to be wrong with the machine it's self so they have decided to replace my MacBook with 15" MacBook Pro Mid 2015. will I notice anything difference?
So this is a no brainer, obviously replace it.
However I would try negotiating further first and requesting MBP'16 instead. Just for the sake of its cost: you could always sell it if you don't like it and buy MBP'15 and some beer or anything.
P. S. Pay attention what configuration they're providing to you. If your late 2013 contains dGPU their mid-2015 might not as currently Apple does not sell mid-2015 with dGPU. If this is the case, at least request some other upgrade to compensate this - like bigger SSD at least. Or, again, as I said - MBP'16.