Welcome to Pherotalk! 
Those two are a good start - Ammo is just what you need to project an all-round sexy/social guy vibe, whilst A314 is brilliant for status in the workplace. They can be used together socially for a much more present sexual/status feel too.
AD has an excellent warranty on their consumer products, and will refund most amply if you don’t get results, all they ask is that you discuss and maybe follow some advice first. So don’t be concerned there.
Yes, it is possible to OD on pheros, which can get you ghosted/ignored or provoke agressive/competitive responses. However, Ammo is about the best balanced none product and it is fairly hard to OD on if you do your basic sweet spot (effective dose) testing properly. Similarly an A314 OD is most likely to have you bosses testing you or becoming scared for their jobs rather than anything bad, whilst socially, you can get taken as too-good, standoffish or a bit dad-like/older than your years. You might find IG a bit more commensurate with your age here. Instant Shine is also another you ought to consider - Happy/feelgood and socially confident - about the most all-round phero that AD sells. Goes well with A314 and brilliantly with Ammo.
Yes, strength is the only difference - the /b version is formulated to be closer to A314 Oil in impact whilst the /a is stronger/more present and higher impact. I’ll say now that I rate the oil far above either spray for a whole lot of reasons but for starting out, sprays are fine, just do the legwork to get you doses right.
Strength is not always good with pheros - you need tofind the dose that keys-in most effectively with your personality and natural phero signature. The common mistake that many make is to pile-on far too much, then wonder why they get ghosted/started-on or find they are only attracting the grannies - too much none! 
Have a read of this - all the basics on choosing and using pheros effectively are covered: