Looking at the technical side, the choice between these two really comes down to how much you value raw optical performance versus convenience. The Canon RF 15-35mm f/2.8L IS USM is generally considered the superior choice for the 45MP sensor because it requires less digital manipulation. The 14-35mm is a clever design, but it has significant native distortion at 14mm that the camera has to stretch out. A few points to consider:

• Canon RF 15-35mm f/2.8L IS USM: Higher edge-to-edge resolution and f/2.8 is vital if youre hoping to catch the aurora.
• Canon RF 14-35mm f/4L IS USM: More compact and offers 14mm, but the corner sharpness can suffer slightly due to the heavy software correction.
• Canon RF 70-200mm f/4L IS USM: Might be worth considering for those distant mountain peaks to complement your wide angle. The 15-35 is likely the lens that will satisfy your pixel peeping the most. It feels more balanced on the R5 Mark II body anyway.

Like someone mentioned, it really boils down to whether you're gonna be out there at night or just shooting during the day. I've been incredibly satisfied with the Canon RF 14-35mm f/4L IS USM for my travel setup. It handles the R5 sensor just fine if you're stopping down for landscapes anyway. Few things to keep in mind for your trip:

• The f/4 version is much lighter, which you'll definitely appreciate after hiking around waterfalls all day.
• You get that extra 1mm on the wide end, which sounds small but actually makes a difference for those massive Iceland vistas.
• Save the extra $1000 and put it toward a top-tier tripod or some Breakthrough Photography X4 ND Filters because you'll need those for long exposures. Basically, unless you're dead set on shooting the Aurora at f/2.8, the f/4 lens is the more practical buy. I dont notice any real sharpness issues on my R5 when I'm at f/8 or f/11. The camera body corrects the distortion automatically anyway, so the muddy corners people talk about are mostly gone by the time you see the RAW file. If you want the absolute king of glass tho, the Canon RF 15-35mm f/2.8L IS USM is the one, just weigh the cost vs how often you'll actually shoot wide open.

Congrats on the new body! Iceland is brutal tho, so be careful with your choice. For that high resolution, I would suggest sticking purely with the Canon pro glass.

• Make sure to get something with solid weather sealing.
• Watch out for the slower lenses if youre shooting the Aurora.
• Native zooms are gonna be your best bet. Just get any of the L-series wide zooms from Canon and youll be fine.

Been using this for years, no complaints