Here are the components to both bikes. Which is better? The Diamondback is cheaper.
Diamondback Insight 3
Sizes
SM (15"), M (17"), LG (19"), XL (21"), XXL (23")
Frame
Insight 6061-T6 Butted Aluminum Performance Hybrid w/ Formed / Hooded Top Tube, Integrated Headtube, Vertical Compliance Stays, (VCS), replaceable derailleur hanger
Fork
Insight integrated Aero Carbon straight blade, Alloy Steerer
Rear Shock
N/A
Cranks
Shimano FCM590 Deore 26/36/48t
Bottom Bracket
Sealed Cartridge
F. Derailleur
Shimano FDM590 Deore
R. Derailleur
Shimano RD591S Deore
Shifter
Shimano SLM590 Deore Rapid Fire 9spd
Brake Levers
Avid FR-5
Brakes
Avid SD-3 Linear
Gear
SRAM PG-950 9spd Cassette (11-28t)
Rims
20h frt / 24h rear Equation R17 Aero Doublewall w/ machined braking surface
Tires
Kenda Kwick Trax w/ puncture resistant K-shield 700x28c
Pedals
Wellgo Alloy body / cage, toe clip compatible
Handlebar
DB 31.8mm low rise Alloy
Stem
DB 3D Forged 7° 31.8mm
Seatpost
Alloy Micro Adjust 27.2mm
Seat
WTB Devo High Pad
Headset
Integrated "Cane Creek" Size Drop-in ACB 1 1/8"
Colors
Gold
Chain
KMC Z99
Hubset
(F) 20h Alloy QR (R) 24h Alloy Cassette QR
Spokes
Black 14g Stainless Steel
Grips
Avenir Ergo 3-D Comfort
Extras
Owner's manual, Chainstay Protector, Clear Coat, H20 bottle mounts



Trek FX 7.4
Frameset

Frame

FX Alpha Gold Aluminum

Fork

Bontrager Nebula, carbon, SpeedTrap compatible

Sizes

15, 17.5, 20, 22.5, 25"
Wheels

Wheels

Alloy front hub, Shimano RM30 rear hub; Bontrager Nebula 32-hole alloy rims

Tires

Bontrager Race All-Weather Hard-Case, 700x32c
Drivetrain

Shifters

Shimano M390 Acera, 9-speed trigger

Front derailleur

Shimano Altus

Rear derailleur

Shimano Deore

Crank

Shimano Acera, 48/36/26 w/chainguard

Cassette

Shimano HG20 11-32, 9 speed

Pedals

Nylon body w/alloy cage
Components

Saddle

Bontrager H1

Seatpost

Bontrager Nebula

Handlebar

Bontrager Satellite Plus IsoZone OS, 25mm rise

Stem

Bontrager SSR, OS, 10 degree

Headset

Slimstak, semi-cartridge bearings, sealed

Brakeset

Tektro alloy linear-pull brakes w/Shimano Acera adjustable-reach levers
Accessories

Grips

Bontrager Satellite IsoZone Elite, lock-on, ergonomic

How much cheaper is the Diamondback? It looks like the specs might be better on it overall anyway.

it is $300 cheaper. However, the 7.4 22.5" fit me perfectly, and the biggest they have on the diamondback is 21". Will that have any significant impact? I assume not, but I am not sure.

I'd buy the bike that fit. An inch and a half is a BIG deal.

How tall are you?

There is a chance that both the Diamonback 21" and the trek 22.5 will fit the same since there are both XL frame size for each model. The Insight look like it has a more compact geometry so that makes the seat tube shorter. I would compare the geometry between the two but Trek doesn't have there geometry specs online. $300 is a big price difference and while fit is improtant there is no reason to spend a bunch extra when the difference MAY be small. there is usually a number of places like big sporting goods stores that stock the Insight even if its the basic one it will fit the same. I would at least try it out.

thanks for all the replies guys, it really helps. I actually expanded my list of possible bikes and pretty much took out the FX 7.4 because it is so much more expensive than the others. There is a Performance Bike by my apartment that I didn't know existed. They have the Fuji Absolute 2.0, GT Tachyon 3.0 and a couple of Scattante that I am going to check out, all of which are around $550, almost $200 cheaper than the FX and I can try them all out. If anyone has experience or opinions on those bikes (or others performance bike may have) please let me know. Thanks again!