I came from Lexus and Toyota my entire life. I still like them very much. But for this time around with a new car, the IS lineup is just too aged to even consider for me. The base price for the G70 beats the IS, even though it has more features. If you want the AWD in the IS, you'll get a 10 year old 6 speed transmission, not a modern 8 speed with better performance and economy.
The dealership experience is likely where you'll notice massive differences. Lexus has the dealer experience down pat, for decades. Genesis OTOH, really has no idea what they're doing with the dealer network because you have go through a traditional Hyundai dealership that has a Genesis "dealer within a dealer" model. They treat you like a number, not a person. Now, Genesis will get their sh*t together in the coming years as they release SUVs.
But in the short term, people who are Lexus, BMW, Mercedes owners would never set foot in a Toyota or Hyundai or other "mass market" non-luxury brand. Other than the car guys and gals who know what they're doing and understand the greatness of the G70 have endured the quasi-Genesis-but-mostly-Hyundai dealer experiences, which is NOT a luxury experience. This means the market base for Genesis customers will be limited until they have actual stand-alone dealerships that offer a true luxury car buying experience. You should use the fact that you are going to a Hyundai dealer to get a Genesis against the dealership(s) you negotiate with, I did and it worked well. Their Sales Manager was speechless when I told him that I could lease a similarly equipped BMW, Jag, or Audi for LESS than a G70 even though the sticker price on those are 10-15K higher. I showed him the math and said, "In order for this deal to move forward, you need to drop the MSRP by 5K as a STARTING point before any OEM incentives, trade-in factor, etc. Why? Those cars have much higher residuals. So while the G70 has a higher depreciation vs. the IS, if the car is similarly priced with similar spec, the G70 has a lower resale value, which means you have more bargaining power to lower the window sticker on the G70 in order to be more competitive with an IS that has less depreciation over "x" number of years. Depreciation makes a huge difference in payments when doing a lease, and I used this to my advantage when I got my G70. I know you're a buyer vs. leaser, but the principle remains.
As far as reliability, I'm not concerned frankly. Hyundai has improved quality vastly over last 5-10 years and continue to do so. Plus Genesis has a longer warranty, 3 years of oil changes included, and valet service all of which is part of the ownership experience.
I think the interior quality and design, and overall features on the interior is fantastic. I can compare directly to my last IS250, and the G70 is every bit as good, actually better in most areas IMO (better Nav system UI), better transmission and AWD systems (the G70 2.0T AWD has better EPA ratings than the IS300 AWD, engine has more torque and better 0-60), and I think the G70 is way better looking than the current IS (the gaping Lexus grille is a huge turn off for me these days).
Ride quality? I live in Chicago, our streets looking they've been shelled by rebels for years. But the ride quality and handling are great, and the steering feel puts the G70 notably ahead of the IS. My G70 is the 3.3T AWD Sport with the 19" offset factory Sport
rims and summer tires, nearly impossible to drive in Chicago winter. So, I got a set of 18" matching
rims with more sidewall on good all season tires to better handle the potholes and snow during winter months, and now I can actually rotate all season tires!
That's my 2 cents having been a Lexus and Toyota driver for 20+ years. I think the G70 is better looking, a better value, a better warranty, better performance and handling, and reliability is no concern for me after my wife's 6 years with her 2014 Hyundai Tuscon AWD limited which has been bullet proof for 80K miles to date.
Just take your time to shop, get dealer quotes, and drive off with good deal on a great car! Cheers and best of luck, keep us posted! This forum is an amazing resource for info and full of genuine car enthusiasts.