What bike is better 2010 Trek FX 7.5 or 2015 Trek FX 7.2? My budget allows for maxing out at $500. The LBS sells the new 2015 Trek FX 7.2 for $450. I also found an excellent condition 2010 Trek FX 7.5 for roughly the same price. Assuming the used bike is in excellent condition with less than 300 miles of use, should I be worried about the 5 year model difference of the two bikes?

I'm upgrading from a 2008 Trek Navigator 3.0 that is too cumbersome for longer rides. I ride trails with the kids, charity rides, and occasional commuting. 

These are the specs as specified by Trek's website. I realize the 2010 FX 7.5 has better specs but should I be worried about their age? Or with these bike components is usage more important than age?

2010 FX 7.5

Frame: FX Alpha Black Aluminum
Fork: Bontrager Nebula, carbon w/Clix dropouts
Wheels: Bontrager SSR w/Clix
Tyres: Bontrager Race Lite Hard-Case, 700x32c
Shifters: Shimano Deore trigger, 9 speed
Front derailleur: Shimano Deore
Rear derailleur: Shimano Deore LX
Crank: Shimano M443 Octalink 48/36/26
Cassette: SRAM PG950 11-26, 9 speed
Pedals: Alloy body w/alloy cage
Saddle: Bontrager H2 Flex Form
Seatpost: Bontrager Nebula
Handlebar: Bontrager SSR OS, 25mm rise
Stem: Bontrager SSR, 10 degree
Headset: Aheadset Slimstak w/semi-cartridge bearings, sealed
Brakeset: Avid SD SD-3 w/Tektro adjustable reach levers

2015 FX 7.2

Frame: FX Alpha Silver Aluminium, DuoTrap S compatible, rack and mudguard mounts
Fork: High-tensile steel w/lowrider mounts, CLIX dropouts
Front hub: Formula TK31 alloy
Rear hub: Formula FM32 alloy
Rims: Bontrager AT-750 32-hole double-walled alloy 
Tyres: Bontrager H2 Hard-Case Lite w/puncture resistant belt, 700x35c
Shifters: Shimano Altus EF51, 8 speed
Front derailleur: Shimano M191
Rear derailleur: Shimano Acera M360
Crank: Shimano M131, 48/38/28 w/chainguard
Cassette: Shimano HG31 11-32, 8 speed
Pedals: Nylon body w/alloy cage
Chain: KMC Z7
Saddle: Bontrager SSR
Seatpost: Bontrager SSR, 27.2mm, 12mm offset
Handlebar: Bontrager Low Riser, 25.4mm, 15mm rise
Stem: Bontrager SSR, 25.4mm,10 degree
Headset: VP, semi-cartridge bearings, sealed
Brakeset: Tektro alloy linear-pull brakes, Shimano Altus levers
Grips: Bontrager Satellite
At the level of bike you are looking at (2010 Trek 7.5 FX), most of the components will be easily found at your local bike shop or online. At this level, most manufacturers will use standard components which are easily obtainable, mostly because using any non-standard components would end up costing more, as they would only be produced in low quantities. Also, because they use standard components, and many other bikes use the same components, there is a big market for replacement parts, which means that finding replacement parts should be easy and inexpensive.  Assuming the used bike is in good condition, you shouldn't be worried about being able to find replacement parts when the time comes. As you move up to more exotic/expensive bikes, you'll start to see more unique parts that may be harder or more expensive to replace on older models.