Viewing Bikes

I wouldn't bother going to see the first bike without the service history being available. If the other is mint it will likely be clean with no obvious issues on show, so I would check out the person selling the bike. Do they look like they have thrashed the bike or not?.

 

Maybe try to ascertain where the 2014 bike was serviced and if possible enquire what work has been done. The engines are Bullet proof but gearboxes can suffer if a quick shifter has been fitted. It’s helpful if the suspension linkages have had some attention. A good general inspection of the extremities like bar ends, footrests and bodywork may indicate if the bike has been on its side (standstill or moving). Does the bike look loved, is the chain clean and adjusted and have a good look underneath to see how it’s been cleaned. My 2015 bike with 13,500 miles looked like new under the engine cos I used to remove the bellypan when I washed it. Valve clearances are due at 12,000 (and every 12,000)and mine didn’t need doing (one inlet was on the tight side and I’d say at the next check it would need doing, bike since sold). Mot history can be checked online and can show any advisories which can indicate how the bike has been cared for.

 

Just the service book missing, or owners handbook too? They're usually kept in the same plastic wallet. I'd be treating with a bit of suspicion, check V5, engine and chassis numbers, got both genuine Triumph keys, lock-stops undamaged etc.
If it has been serviced, where, the dealer would have a record on their database, any receipts?.
+1 all that @Yorkshireman says, eccentric hub for chain adjustment needs greasing regularly too.
If either bike is coming up for a 12k miles interval use this to haggle the price down, a valve service is not cheap if done by a dealer.