Pro's and con's no matter what you consider, but here's my $.02 as an owner of a 2016 GT350 from new.
I'm biased, but as a car, its had more concerns than any other car I've owned.....oil cooler line recall, bumper sag TSB, oil consumption, oil blow-by, possible A/C evaporator TSB, body panel gaps are not the best etc.
The cost of ownership is high ( tires cost a bundle, front and rear are different sizes and they wear out fast, less than 20000 km if you're lucky, less than 10000 km if you're not, oil changes will cost you a $30 filter and 10 litres of full synthetic at $10 to $12 a litre plus labour plus disposal plus taxes, haven't even considered the cost of brakes yet, I'm sure the two piece rotors are cheap lol and the sticky tires throw rocks like a MF ).
Some guys who track their 2016's have had issues with high transmission temps and limp mode, personally, I've never had any issues like that , but then again, I don't bother with the track. You can retrofit a transmission cooler into a 2016 with a kit from Ford Performance, if you think its going to be an issue.
These issues are well known across the board, and pretty much come with the territory for any GT350, no matter which model, so no year is any worse than the other, unless you consider the previously mentioned transmission cooler between 2016 and 2017.
2017's had the transmission coolers added as stock with all the other coolers, but it also added $10k to the base price.
On the flip side, despite all the problems, you'll never find a better bang for your buck......the torque curve is amazing on this car, and the 8250 red line is staggering, and the sound.....good lord the sound of this car with stock exhaust in Sport Mode is an ear-gasm every time you fire it up and run through the gears . No need for any mods, other than maybe the lowering springs, its got the whole package. Easy to achieve 80 km/hr at the end of first gear........or so I've heard.
Best value is a well loved 2016 Tech Pack, but prices are still up there ( +$50k), depending on your friends budget, not a lot of differences in the drivetrain between a 350 and a 350R, maybe the resonator delete, but the 5.2L , six speed etc are all the same.
I've considered selling mine, but not quite there yet.