1) I don’t think that you can go wrong with either of your choices.
2) I expect that all plastic instruments have somewhat of a slippery feel. So do some wood instruments. I solve the problem of slip two different ways: a) for a lighter-weight instrument, I simply lick my right hand thumb to give it some “stick” before playing for a bit; and b) for a heavier instrument (i.e. my Mollenhauer Modern in palisander with an E foot), I wrap a rubber band around the body to serve as a thumb rest or slip stopper if you will.
3) I have had a Zen-On Bressan alto for several years. I like it just fine and use it when I want to give my two wood altos a rest. I can’t remember if I got it from ASW or Boulder, so I can’t tell you if it was “improved” or not.
4) People’s one-or-the-other answers rely on both experience and taste. Experience answers are often incomplete. And tastes can be rather subjective at best, and not necessarily reflective of your taste. Since you have a budget of a hundred bucks or so, why not get both–the Zen-On from ASW and the Encoder from Bill Lazar. That way, you’ll have two slightly different sounding instruments that should satisfy your varying aesthetic desires without breaking the bank. All you will be out will be double the postage charge.
5) Finally, if you have not already done so, you might seriously want to join the various recorder groups on Facebook–a) flauto dolce, recorder, flute a bec; b) flute a bec; and c) recorder players n enthusiasts. They are all extremely active and you will undoubtedly learn a whole lot of stuff and find that any questions you might ask will field more answers than they do here where crickets seem to abound. (I am somewhat of an anomaly since I like to be helpful and tell people about stuff that interests me.)