I have been considering building a light ATB type bike out of a 90's MTB frame. I have been looking for something all aluminum in my 23" frame size but nothing has surfaced in a while. I found a Specialized Hardrock non suspension all steel frame from the 90's in my size for $125 locally. It has been kept in the dry and the frame looks pretty good. I had originally thought a Rockhopper had a better frame, but I might get this Hardrock if there isn't that much difference in the frames.

There were many different levels of Rockhoppers and Hardrocks back then; if it's a 'base' Rockhopper then the frame won't be much different than one of the upper-end Hardrocks. Do you know what level of Hardrock you're looking at? Also pictures would be helpful so we can help you identify the model and frame tubing.

There is a difference. Some early rockhoppers have triple butted frames. I think the Hardrock was at or near the bottom.
I would hold off. But that's just me.

I just purchased a Hardrock for my son to bang around on. Only $40, but needs a new rear wheel. The frame is all steel.

The Hardrock is below the Rockhopper in the line up, at least some years it is straight gauge Cro-mo tubing when the Rockhopper is double butted. There will be a difference in components. I don't know on Specialized specifically (too lazy to check catalogs) but what I have found with older mountain bike is lower end bikes no longer have stainless steel spokes so they rust, as you get farther up the line, not an issue. Edit: Re-read OP, clearly said "frame"

Thanks guys. I will find out if it is at least a double butted frame. Maybe that aluminum frame will come along soon.

IMO, $125 for a frame is too much if it's not super high end or rare.

The nice thing about HardRocks is that they're the bottom end of a good quality line, and once built up it'll happily take all the abuse you can dish out.

Spec’s lineup has always been HardRock, RockHopper, Stumpjumper, with the Stumpy at the top.
There’s always been a couple of different “trim levels” of each model, and they’ve used those nameplates for decades, so there’s literally hundreds of different bikes under those three names.

Generally, I’d say that a Hopper would be a better grade frame set than a HardRock of the same year, but you’d have to dig into the specific catalog for the year of the bike you’re looking at.

It may be like Cannondale, where there were only one or two grades of frame, shared among several different models; distinguished by the wheel/fork/component builds.

Another thing to consider is that -- and this only my opinion -- a steel frame, even the lowliest Hardrock, will have a nicer ride than that aluminum frame you're looking at. Of course on a vintage MTB that's all relative as you can put somewhat fatter modern tires on either and they will have a pretty cushy ride.

It's a complete bike in good shape for the age. I just meant that I would be changing most parts, so group sets, and such didn't matter to me as much as the frame.

I'm pretty heavily invested in 26" wheels. I own 5 bikes with 26" wheels. I have ridden an all-aluminum frame 26" wheeled bike on the forest roads I mostly ride, and tires will provide enough suspension. My aluminum MTB is too small but would be great in a larger size. I'm bad to buy a bike just for parts. I bought 2 Marin MTB with Aluminum frames for $100 because they had good 9 speed Deore components and wheels. I rode the 17" one a bit even though it was too small for me at 6'2".

It’s not the frame material. Cheap bikes ride like cheap bikes. The straight-leg oversized cro-mo forks everybody put on mid-range rigids didn’t help much either.

Mid-1990s I would avoid aluminum unless it’s Cannondale or KLEIN, they were way ahead of the game, most other mfgrs were just making aluminum copies of their price-point steel bikes, just a little bit lighter unless you went way upmarket to something like a Technium or an M2 StumpJumper

I have a Klein Pinnacle with a Spinner fork that is a fantastic rider, and a very capable all-surface bike.

if you are building from scratch I would opt for a better frame - consider a Stumpjumper frame over a Rockhopper or Hardrock if you are targeting vintage Specialized steel frame

or other options in higher end steel frames - including Gary Fisher, Schwinn Paramount, and Trek

Quite a few years back I built up a HardRock as an all-purpose commuter, hauler, beater. It lived for a few years under a tarp in my backyard, which was an interesting experiment in rust. It fulfilled its intent, but it was heavy with a deadened (as opposed to lively) ride. I eventually lent it permanently to a local friend whose commuter had been stolen, and last I heard it had been stolen, too.

It's still a bit much for a HardRock, but not that much, and I suppose it depends on your market. If it fits and you like it, get it, and make sure to show us pics!!

At 5'4" I'm also a big fan of 26" wheels. They fit small frames better and are more nimble than the big wheels.