Replacing a 12ah battery with a 14ah isn't a problem - so long as the physical dimensions and cable mounts are ok. Usually, a larger ah battery is physically a bit larger. The 14ah battery will charge the same as the 12ah battery, but will take longer to get to full charge. It can handle a larger draw from your electrical system (because of a larger cold crank rating), but the larger battery itself isn't going to put an additional load on it. It will also be able to supply power past the point where the 12ah would be dead because of the larger reserve (ah).
The downsides are additional cost, additional size and weight.
The stator capacity on the carbureted and FI XCiting 500's are the same.
I thought it odd that Yuasa would name 12ah batteries with 14 in the name instead of 12 - but I saw them listed online as 12ah batteries. After looking through Yuasa's site - the info on the YTX14's states that they are "Capacity: 12", with "Amps: 1.4". You would think that the capacity was ah, but that doesn't jive with 1.4 Amps, or with the 14 in the name. I don't know what the capacity is - because being able to provide 1.4 amps an hour means it is a 14ah battery. Their listed difference between the regular and high capacity is with the cold cranking amps (200 vs 240).
The lithium Ion's are really nice - but cost about $240. They last about twice as long as lead acid, weigh 1/3 as much, hold a charge longer, and deliver cold cranking at about 315 amps - but they are affected much more by cold. At 40f degrees you need to "wake" the battery by running your lights for 30 seconds before cranking the starter. Add another 3 seconds for each degree below 40. On a sportbike reducing the weight by 6lbs would help justify the cost - but I don't see them being worth it on a scooter.
AGM or a Gel would be my choice. Gel's have less cold cranking amps than AGM's, but are less affected by heat. Gels also don't work well with large stators - but that isn't a problem with an XCiting (I wouldn't recommend them for a BMW GS though).
