Both the phones come with the same specifications but I will recommend you to go for OnePlus 6. The OnePlus 6 is lighter as compared to the Vivo Nex (by around 22g). The OnePlus 6 offers better camera i.e.16 MP + 20 MP Dual Primary Cameras and 1.7 F Physical Aperture for better low light photos. And also, it supports NFC and DLNA (DLNA is used for connecting to a DLNA TV over WiFi).
I will recommend you to go for the Galaxy S9 Plus. The S9 Plus comes with a screen resolution of 1,440 x 2,960 pixels which is the highest amongst all (other three phones). It is the only phone (as compared to the rest of the three) which supports expandable storage. The cameras on the S9 Plus offer a dual aperture of f/1.5 and f/2.4, and can automatically switch between them depending upon the ambient light. Inside the smartphone, there are two Quad-Core (2.7GHz M2 Mongoose and 1.7GHz Cortex A53) processors. A stylish looking smartphone from the topmost brand with such great elements makes it a perfect buy.
I will recommend you Vivo NEX. It comes with a faster CPU and more RAM than the Note 8 (Qualcomm Snapdragon 845 Octa-core 2.8 GHz, Quad core, Kryo 385 + 1.8 GHz, Quad core, Kryo 385) and it comes with more onboard storage. It is slimmer and still manages to pack a bigger battery (4000mAh). The Vivo NEX is an excellent smartphone which comes with some exciting features. Overall, this is a perfect smartphone amongst the two.
I will suggest you go for Oppo Find X. Although these two smartphones share the almost same specs, the Find X comes with a better camera than the Nex. The Oppo Find X is armed with a dual primary camera setup with 16MP and 20MP lens. The camera setup can record 4k videos at a speed of 30 fps and has CMOS sensor and Phase detection autofocus. Find X also has better storage options along with expandable memory card slot. This massive storage helps you store everything you want to. Moreover, the addition of a quick charging reduces the wait time before you get back to action. It also supports NFC.
I will recommend you Vivo NEX. It comes with a 6.59-inch Full HD+ display with a screen resolution of 1,020 x 2,316 pixels. The performance of the device is backed by Qualcomm Snapdragon 710 (2.2GHz Kryo 360 Dual-Core and 1.7GHz Kryo 360 Hexa-Core processors) aided by 6GB RAM. Equipped with a powerful 4,000mAh Li-Polymer battery, Vivo Nex is an excellent smartphone to buy.
I will recommend you Vivo NEX S. It comes with a better processor and more RAM.
It is powered by two Kryo 385 quad-core processors are present that have 1.8GHz and 2.8GHz clock-speeds respectively. The processor is further assisted by an 8GB RAM and an Adreno 630 GPU. The combination delivers a quality performance and kit ensures that you can play heavy games as well as run multiple tasks.