M2 is very fast, but sata 3 SSD are still light years ahead of standard hard drives. Samsung 850's you can get pretty reasonable too. You could always spend the cash on a PCIE SSD.
If you can, wait and save up for an M.2. The newest M.2 cards are around 4 times faster then a SATA SSD. Currently the newest M.2 are close to three times the price of a SATA SSD. But I would assume there will be a price drop in the future. You could always get a used Samsung 850 EVO 250GB of eBay, to use for now, then sell it off when you get the M.2.
Hi there Depends on size and if yours motherboard supports m2,then I would go with m2,500gb can be bought for reasonable price and if my motherboard supports than I would probably go with this,but too new nVMe format is becoming more used and not sure if I would bit wait and get nVMe or PCIE SSD,hard to decide there Hope this helps Thanks,Jura
