If you are talking about features of the MBP itself, as long as you are not looking at the Retina models, then the memory you can upgrade yourself for a lot less than Apple charges, and that does not affect the warranty as Apple considers memory a user permitted upgrade. Great sources are Crucial and OWC, both are Mac specialists and have on-line videos on how to do the install.
If the 15" is what you are looking at, a good upgrade is the 7200 rpm hard drive, gives a little better performance over the standard 5400 rpm drive.
The Hi Res screen is good if you do a lot of graphics work.
Comparing the 13" amd 15" you will notice the 15" has a second, discrete graphic processor...helps when you get into heavy duty videos, games, graphics...but keep in mind that kicking in the second processor uses a lot of power and will run the battery down faster.
If you are looking at the Retina models, then keep in mind there are no post-purchase upgrade options...you have to buy with everything you want at the start.
Just some thoughts.