We've had our GLC for about 6 months and 17,000km now. When we were in the market, we didn't drive the Macan as our son is too tall for the back seat.
That being said, every time I see a Macan I love the overall exterior look of them a bit better than the GLC and given how rare the back seat gets used, have always wondered if we made the right choice.

Recently we were in San Diego for a few days and rented a 2017 Macan S (and a 2018 Cayman.)
(I know the Macan GTS would be a better comparison)
I'm not sure what options the Macan had other than the 20" rims as I don't know what come standard on them.

That being said, short version I was worried I'd love the Macan and regret buying the GLC. It was the exact opposite.

The Macan (I know it has a bit less power) felt lethargic in comparison. It seemed like there was a ton of turbo lag and the 7spd gearing was too long. The 9 speed and engine in the Macan seems much more responsive.
The ride itself was a bit smoother feeling (I prefer a stiffer ride, totally fine driving around with suspension in stiffest setting)
I was let down by the interior...it just isn't as refined. Very comfortable, nice seats, but overall just not as nice as the Mercedes. The "dome" light is like a fluorescent tube. The arm rest looks cheap. Sooo many buttons. Prefer the Mercedes instrument cluster.
Rear visibility is very poor due to the roof slope. (You're view is limited to the space between the rear headrests.)
I don't know if it can be adjusted, but the lane warning and parking sensors are way too sensitive. It was constantly making noises that I was too close to the lines and when parking is beeping like crazy. As soon as you'd put it in reverse, it would be beeping from 6ft away.
The Macan sounds better (inside the vehicle) when idling, but the Mercedes sounds better while driving/accelerating.
There are things I don't love about the GLC (mainly around using the Navigation and other interfaces) but overall I walked away from it extremely happy with having purchased it.
I'm curious to see what the Macan is like when the new model comes out in a few years or whenever it's supposed to be.
The Cayman on the other hand...what a fun little car! (Even being a base model.) Apple car play was awesome too.

 

I recently test drove a 2018 GTS. I feel that the interior quality of the Porsche is better than the Mercedes though I do not like the cup holders on the Macan. I still don't get why Mercedes still uses cheap door pins (I know I can replace them with fake AMG ones) and not a fan of the display. I wish the new display would be out sooner. I like the way the GLC looks from the side and back. Don't like the fake 3-D star for those vehicles that have the sensor for pre-crash. The GTS was too quiet (even my wife thinks so-she daily drives the C63). The Macan feels "snug" inside while the GLC is "airy" in comparison. In the end, I'm leaning more towards a GTS though the GLC43 is a better bargain.

 

Yeah the Mercedes emblem for the presafe should be bigger.
The cupholder/armrest area I didn't like either. It felt like something out of a less expensive car. I've read that the GTS has a nicer interior than the S? I'm not sure what the differences are though. Price wise I believe the entry level S and GLC43 are right around the same.
I do like the exterior looks of the Macan better though...yesterday I was parked next to one. Sounds great when it starts up. As it drove off, I said to my wife, "wow the GTS sounds great, way better than the S" and then realized that it was my brother in law driving by at the same time in his M4 with competition exhaust, lol.

The Porsche transmission is better at low speeds too, none of the jerkiness that the Mercedes has especially in Sport+

 

The GTS does sound different over the S because it has the "Performance Exhaust" as a standard item that is part of the GTS package. Though, the M4 probably drowned out the GTS.