please excuse my misinformed friends....

ALL trek 800 series MTB frames are STEEL...

there are model year differences in the two bikes you purchased.

both frames use low end steel.

i am not seeing any indication of "frame/Crash damage to either frame.... the STEERING ANGLES were changed in later models.

the differences between ther two bikes will be:
1) the one with the steeper fork angle will steer a bit quicker/easier, but will be a bit less stable when riding down very steep hills...
2) the parts on the 820 will be of a slightly higher end group... this is designated by the "820" model number... all trek frames of an 8XX series within a particular year will have identical frames... the bike with the larger tubing is a newer model... if the deralleurs are the same, that just means trek chose to upgrade the later model 800 to a slightly better parts group...

the 8xx treks are third in order in the trek MTB line of STEEL MTB's... they are rugged and dependable frames that hold up well to abuse... in good working order they sell for 75 to 125 dollars US.... some sell for $150 if in nearly new condition.

Trek MTB bikes with FOUR digit model numbers have aluminum frames, THREE digit trek MTB's have STEEL frames.... 3700, 4300, 4700, 4500, etc. are aluminum framed... 220, 7xx, 8xx, 9xx have STEEL frames, period. First number is the frame type, second number designates the parts group installed at the factory....

the "thicker" tubing lends flex resistance to the frame, and not much else, other than looking more high tech, of course... tubing QUALITY would probably be identical.... the WEIGHT may indeed be lower with the "thicker" tubing... stiffer frame designs allow for reduction in tubing wall thickness... whether trek decided to thin out the tubing is a question they might have answered in the past.... weighing the bare frames would be a way to find out... not worth the effort, IMO... both frames are stout, and barring extreme abuse, they wont break.

if one of the bikes will be for you, choose the 820... give the kid the quicker/easier/newer steering 800.

OH, and do install the seat on the 820 correctly to the seat post... the clamp SHOULD be to the REAR of the bike... looks like it's now pointing forward... and set the seats LEVEL to the ground... the way they are angled now will cause pain.... and the twist type shifters on the 820 suck, IMO... upgrade to CLICK shifters like the 800 has, if you want and can afford... you will not be disappointed.