Here's my best shot at answering your questions:
1. Based on my experience and my ENTs recommendation, I switched from the WaterPik (which I used for 2 years) to the SinusRinse bottle and found that the SinusRinse bottle works better for me (although there are people who swear by the Grossan/WaterPik and who would tell you the opposite) - the Neti Pot doesn't work as well because it relies on gravity while the SinusRinse bottle allows you to spray the liquid up into your sinuses. For me the SinusRinse bottle gets a larger volume of liquid into the sinuses and is much cheaper, simpler, more convenient and quicker to use. The WaterPik was fine but it shoots a narrower stream and it seemed like you had to aim it in the right place to get the liquid into the nooks and crannies, plus the WaterPik attachment tended to cause some nosebleeds in places it touched inside my nose. My ENT recommended the WaterPik for several years but switched all of his patients to the SinusRinse bottle when they became widely available.
2. I'm not taking either right now but I preferred Zyrtec to Allegra - it seemed to be a little stronger (and the fact that Zyrtec is now available over the counter so you don't need a prescription is a huge plus in my book - I guess that depends on how much you pay for a prescription).
3. I can only tell you what my doc told me which was "the best cure for your symptoms will be cold weather". If your symptoms are pollen allergy induced you should be pretty safe this time of year - and one of the downsides of taking antihistamines in the winter is they can dry out your sinuses even more during this dry time of year (which can cause the mucus to thicken and the tissues in your sinuses to become more inflammed).
Hope all of that helps.