As there are 2 options for sim
1 x sim + 1 x sd card (single sim)
1x sim + 1 x sim or sd card (dual sim)
As dual sim gives you more option why would anyone choose single sim as the price is the same?
Dual sim s9+ for me :-)
@S8Owner76 it's a good point, and I think the dual-SIM option is very popular.
As you mentioned, the single-SIM S9/S9+ has a micro SD card slot, where you can add up to 400GB of extra space (for photos/videos/music etc).
Link for reference > http://www.samsung.com/uk/smartphones/galaxy-s9/performance/
I'm confident you'll be really happy with the phone, regardless on which option you go for!
Yes I like the sound of Bixby @Ramz and I think AI is the way forward for phones now, and the camera looks amazing, but I've always felt Samsung are brilliant with their cameras, but are definitely getting better.