If you can't afford both yet you buy one that's because you're making an investment, and the GT-R is the better option because it's rarer so it will appreciate more than the Senna.
That doesn't necessarily make it a better investment. Time/availability is the key factor with car investments.
Right now, because the production is still fresh, Sennas are trading around £1,499,995 and up, double its £750,000 MSRP. MSO options build even stronger value & McLaren doesn't regulate its VIPs like Ferrari/Porsche, so you can flip immediately without the manufacturer black listing you. In a year or so, a Senna could likely fall back down to £900,000-1,000,000 because more are potentially available, but its capped production & halo status will always retain value. If you were desperate to move one after 1-2 years, asking £800,000 would ensure an almost immediate return on your money.
The GT-R50 isn't out, but based on ItalDesign's last car, the ZeroUno, rarity will push a higher value, but you're going to sit on it a lot longer due to certain traits. Potentially, 6-12 months longer which open up other risks against your asking price. The ZeroUno was limited to 5 coupes & 5 roadsters. Coupe #002 sat on the market for a year at $3 million, with original MSRP at $1.4 million. Joe Macari had one coupe listed in August with the listing gone, but the car still seen there as of November. Coupe #001 was listed in August as well and is also still available with P.O.R.
The thing is, the ZeroUno is not a particularly pretty car and it's built on a R8/Huracan, but there's only ten
built, so its super rarity is what drives its huge market value. The GT-R50 is a much nicer car to look at, but it's intended to be built in 5x the the amount of the ZeroUno and ItalDesign hasn't drastically changed the interior design; it's a very, very nice refined GT-R interior. I still have no doubt people will shell out $1 million for it because it's the latest coachbuilt car on the market, but the question is, will the secondhand market
shell out the same? It's still bearing Nissan/GT-R nameplates at the end of the day, and this is no longer the $70,000-$100,000 market Nissan was able to succeed in b/c the performance undercut everyone else. It also won't have the luxury of the ZeroUno's super rarity if we see 3-7 of these GT-R50s crop up on the second hand market within' a year or so of each other when deliveries begin, which will affect any attempt to ask for a 100% profit. This assuming ItalDesign actually moves all 50 cars, though. I have a feeling maybe only 30 or so top will actually be built.
So, as far as the better option, cars are always awful investments unless you intend to get in early and sell before the market can saturate with something hot. That's the Senna. Brand name halo car, sold out immediately, very early production right now means instant return on the money. No fear of retaliation from McLaren, either. As far as I've read, the biggest factor against the GT-R50 returning your investment is that it's not sold out which means you'll only be able to attract those who want one, but don't want to wait if you try to sell one early. Everyone else will likely bite the 2-year waiting list. The other option is Long Term investing and that will always be a huge gamble with the GT-R50 due to uncertain factors right now. The Senna can right away, fall back on its sticker price if you want to use & keep it because again, brand name halo car with limited production & a market that will likely never grow behind more than 10-12 at a time available.
Why buy this if you can get the McLaren Senna for 100k cheaper?
They're really targeted towards different markets.
The Senna's selling point is that it's the fastest track car you can buy for the road. That alone should indicate how exactly it may drive on the road, despite what McLaren does to refine it as much as possible. It more than likely transfers every bit of road surface & imperfection to the driver b/c well, that's what it was designed to do on the track. The GT-R50, as much as its built on a track-capable car of its own, doesn't take such extremes. The R50 is a coachbuilt GT-R, so the chassis & suspension are still far more forgiving and ItalDesign hasn't changed the interior drastically, either; it still sports big, leather/alcantara seats that are far more comfortable than the Senna's Formula 1-esque carbon molds with some alcantara patched on them.
It really depends what you intend to do with them.