Okay so I've had a K04 MK5 and I now have a JB1 MK7, so I think I'm in a pretty good place to give my opinion on this
Haha. Also let me preface this with the obvious: You are on a MK5 forum, of course opinions will be biased towards the MK5. Head over to the MK7 boards and opinions will be equally biased towards the MK7's favor.
Just for reference I have a Base 2016 GTI 6MT S and I had a Base 2007 GTI 6MT. Anyways here's some things that I have found between the two:
1. The MK7 is a better car in every possible quantifiable way. It is lighter, faster, very tunable, a nicer place to be inside, it handles better (More on that later), and it is easier to work on (More on that later).
2. The MK7 so far has undefeatable electronic nannies like torque management and ASR traction control. (A big negative)
3. The MK5 gave me more "fizz". That unexplainable feeling that makes you really enjoy driving a car. It feels more raw and unrefined than the MK7. The mods I had could have had a large effect on this.
4. The MK7 is MUCH more maintenance friendly. It is a joy to work on. DP is V-banded, cleaning valves can be done start to finish in less than an hour, everything is more accessible etc.
5. The MK7 handles quite a bit better stock for stock. It rotates much better than the MK5 and even without the LSD I feel like corner exit speed is a good bit higher.
6. The PP is probably the best option that has ever been offered on a VW to date. They give you an LSD and the R brakes for only a small amount. The R brakes with the new GTI's weight put the PP GTI in a braking bracket shared by some SERIOUS performance cars. Coupled with some race pads and fluid, I think you would have a real track weapon.
7. It is a new car with new car features and warranty. Bluetooth calling works great and is genuinely one of the best things I have ever had in a car. I can literally leave work early, and call into meetings while wasting no time and not being unsafe to those around me. Even with the windows down people can hear me loud and clear. In fact, I have never ever had a single person ask if I was in the car while using it.
8. For some reason the MK7 seems to heat up amazingly fast. It is brilliant. Along with the heated seats that will light your ass on fire.
9. The MK5 felt a bit more capable during winter. This could be due to the extra weight over the front axles.
10. Fit and finish is great on both but the MK7 is a huge upgrade over the MK6 interior, and a small upgrade over the mk5 interior. You seriously have to look very hard to find any hard plastic. There is a strip of plastic on the doors, but that is it. All touchable's are fantastic quality. NVH is low and road noise is lower than the MK5.
11. MK7's are crazy tunable. With the new revisions of tunes coming out, stage 2 with literally just a downpipe and a tune are putting down an average of 315-320 whp. There's a dyno of one putting down 340whp and 394wtq with just a catless DP and tune. Stock airbox. That is absurd.
I can go on for days but I'll spare you. If you have any questions ask here or PM me. (I used to test drive cars at FCA so I am extremely keen on the details). Now here's my actual opinion: I would wait. If I were you and you are capable of doing the work yourself than go K04 on your MK5 for a while and buy a MK7 after you've had your fun. The price depreciation on MK7's right now is CRAZY. I'm assuming this is because of dieselgate. My 2016 is worth about $6,000 less than I bought it for 3 MONTHS AGO. That is unheard of depreciation rates. It may be worth it to buy a pre-owned MK7 and let someone else take the initial value hit. Keep in mind you want a 2016 since it has the MIBII infotainment system w/ apple car play. I love my MK7, but I appreciate my MK5 even more now for it's flaws that gave it character. The MK7 is perfect, too perfect and that's why I don't get the same "fizz" feeling about it. Shoot me any questions you have. Cheers.
TDLR: K04 the MKV, drive it for a while, part it out, buy a certified used MK7. And get a manual for god sakes