@Vadirocks M51 is kinda mix of Samsung A and M series. Advantages of Samsung M51 over Nord.
[1] M51 comes with AltZ life / Secure Folder powered by Knox. This is number one reason for me to go for Samsung phones.
[2] Bigger 7000mAh battery and obviously better battery life.
[3] Better cameras, both back & front.
[4] M51 has Plastic frame + Plastic back. Nord has Glass back, but with plastic frame, a bad combination, imho (explaination below)
[5] Slightly bigger Super AMOLED+ screen (no pentile matrix afaik). 6.7" vs 6.44"
[6] Has dedicated SD card slot for further storage expansion.
[7] Has 3.5mm audio jack and even FM Radio.
[8] Single punch hole selfie cam, instead of Pill shaped dual front cameras, which is an eye-sore, imho.
[9] M51 is non-chinese phone, if you care..
Reg. Frame and back of smartphones:-
Metal is for sturdy and is the best for frames as well as back. Its Only con is heavy.
Plastic looks cheap, but slightly flexible and non-fragile and light-weight. Best for phone back and still usable in frames.
Glass looks premium, but fragile and not at all sturdy, has heft as well (comparing plastic). Glass is OK for backs and obviously can't be used for Frame.
MetalFrame + MetalBack > MetalFrame + PlasticBack >> MetalFrame + GlassBack >= PlasticFrame + PlasticBack >> PlasticFrame + GlassBack
Plastic frame + Glass back is a terrible combination. Because the flexibility advantage of plastic frame is going to kill glass back faster than metal frame.
This is the reason I am using Samsung Note 10 Lite and S10 Lite, as they both have metal frame + plastic back combination. I really miss the metal back combo of my current backup phone Moto One Power.
To be fair, here are the advantages of 1+ Nord over M51.
[1] Relatively Compact phone.
[2] Has LTE-CA (3CA, if I am not wrong) and M51 has none.
[3] Screen with 90Hz refresh rate.
[4] SD765 vs SD730G. Better performance + unsure 5G support.
[5] UltraWide selfie / Groupie front camera, even though its just 8MPx.
Things that are EQUAL, imho:-
[1] WiFi ac support.
[2] Capable Fast charger in box. (25W === 30W)
[3] Fast fingerprint scanner (Side === in-display) wrt speed and accuracy.
[4] Good SW experience. (O2 === OneUI) [Even O2 OS 11, inspired a lot from OneUI]
[5] Dual VoLTE for all operators, incl BSNL.
I think, I have confused you more than giving you clarity. But I recommend you to get through infos, before opening your wallet...