>>1230008A 90s rigid 26" bike is objectively inferior to a modern 29" hardtail, there's no way around it. The geometries are shit and guarantee instant OTB, small wheels perform poor and modern entry lever 29ers are so cheap there's really no reason at all to go vintage.
>>1229991Rockshox Recon silver is very good. It's the cheapest common air fork available, you usually start seeing it on ~$800 or so bikes.
OP, any reason you want 27,5" instead of 29? Out of those two I'd go with the Giant - the Diamondback does seem to have an idea with longer suspension travel, modern geometry and 1x transmission, but 1x8 with a lightest ratio of 30:32 just really doesn't cut it and I don't think they make big cassettes for 8 speed. Maybe if you're willing to pay a bit for upgrades you could slap 11 speed SRAM NX or Shimano SLX and an air fork on the Diamondback and you'd have a very decent bike, but as it is I don't really consider it usable for MTB. The Giant has very similar modern geometry and is definitely better out of the box, and it's tubeless too so the only thing the Diamondback has going for it is the longer travel (but it's a cheapo coil fork anyways) and being ready for an internal dropper post, but you can just route an external one with zip ties on the top tube with the giant anyways.
