[Samsung Galaxy S9+ Questions & Answers] gsser:
Most important thing is headphone jack and it's quality. S9+ has the best speaker system i have ever seen. With dolby it is way ahead of its game... I tried iphone 7plus for a week and boy i couldnt handle the no headphone jack... It is just cancer to me. Cant use headphones while charging(especially gaming). So i learned a lesson from this, i will never buy a phone without jack. I dont have the kind of budget to buy high quality bluetooth earbuds. Also akg buds are the best i have seen that it comes inside the box. I had razer hammerhead pro v2, akg is way better than that. I also love tuning my sound with v4a so im into sound quality more.

S9+ will rock with custom roms no matter what. I also hate notches... It is like a blank space that i cant use the screen fully.

Best things to look for before getting a phone,

Your preferences

Headphone jack

Software compatibilty and development support

Performance

Overall quality

Phone's own headphones

Overall user experiences and reviews

What you expect from a phone

Notch(it may really annoy you, so go to a store and get your hands on a demo device/i got my hands on s9+ before buying it\)

So before you make a purchase or get the device, make a list of important things for you, go and see the devices by hands, compare them, look for custom rom development(you may want to try diffrent interfaces/i like lineageos a lot\).

Dont forget that the phone you will get will be with you for a long time. Pick it like you are picking your soul mate ?.

I had note 8, it was satisfying me a lot, but when i had the opportunity to get s9+ i went for it. In my social areas, most of the people are using apple or samsung. From what i heard i never heard someone with a bad look to samsung phones. I have been using samsung galaxy devices for almost 2 years now.(my dad had note 3 and he had switched to note 8 1 year ago. He changed many phones and he is prefering Samsung more.) Some things may be a pain in the ass but if you are looking for a long term usage, i would say go for s9+. Mi8 is also nice but i prefer s9+ over it. So like i said go see both devices. Touch both devices, feel the phones and listen to what your heart says. (The phone you will pick will be your best friend. Remember that)

Edit, i forgot you had mi8 already, you can just look at other devices at the same prices. Or if your phone just suits you and gets the job done, no need to move. You can do another things for that money. It is like buying a car or buying a house for the same price. Also your deal is a good one. Just 150 euros for a 4 days old s9+... I traded my note 8 and gave 150-200 dollars more for 4 months old s9+. My phone was bought outside of Turkey, Samsung Turkey doesnt cover any warranty for phones outside Turkey and it has a big impact on second hand sales cause of no warranty. I made the choice before it was too late, if i waited longer my phone would lose more value over time. 2nd hand phones purchased outside Turkey prices are around 2600₺ while Samsung Turkey warranty covered devices prices are 3500₺. Got my s9+ for around 3900₺/I knew the seller and my relationship with him is good, he counted my device for 3000₺\( non used s9+ are like 4200-4300₺. )