I have an early 2015 MacBook Pro, which is the smaller, less powerful 13 inch model from that year. There is no such thing as a "late" 2015 MacBook Pro. I really like the 13 inch model because of its great battery life, high-resolution display, lighter weight, and its general reliability, especially at a lower price point than the new MacBook Air. I actually have an extra one I just finished fixing up that I'm looking to sell. I'm just not very knowledgeable about how to sell it.
Now, about your question:
I could list all the specs, but that would be tedious, so I suggest you read them at apple's website:
13 inch "early":
https://support.apple.com/kb/SP715?locale=en_US
15 inch "mid":
https://support.apple.com/kb/SP719?locale=en_US
The thirteen inch model number is 12,1.
The fifteen inch model number is 11,4.
The fifteen inch MacBook Pro has a 4-core processor, more storage and RAM at the base model, is generally "more powerful", and will probably cost you more to buy.
The thirteen inch MacBook Pro has a 2-core processor that is a generation newer, has longer battery life, has slightly faster ram, and is lighter weight, and will probably cost a little less. Make sure you get enough storage.
Both models have the fastest wifi in any MacBook Pro at the moment.
I really like the visual proportions and weight of the thirteen inch model, buy I am biased. I got mine earlier this year (it is my first mac) and have loved it ever since.
I hope this answer helps and was not too confusing. I am assuming you are not planning on doing any DIY upgrades.