I asked for some advice not long ago, and here's how it went.
I bought a new Watson w-889 expandable antenna for my 346XT, and it came in the mail the other day. Closed it's about the same length as the stock antenna that comes with the 346xt.
I am still experimenting with different lengths for different frequencies, but already it does a better job than the standard "rubber duck", but it is not quite as good as the external car antenna (Hustler) I use while driving. But even inside a building, or outside while walking, the Watson pulls in a lot more, from farther away too. Although on that last point, i've noticed some days, regardless of antenna used, it's easier to pull in signals from farther away than other days.
So far, for a noob like myself, I'm happy with it