That depends on how often you think you'll be searching in English or Pinyin. I for one find the keyboard very useful on my Treo 650, mainly because it helps with Pinyin (and mixed Pinyin/character) searches, but if you're mostly buying PlecoDict as a tool for looking up unknown characters then you might not get that much use out of a keyboard.
And if you expect to be switching frequently between the two input methods, you might prefer a Pocket PC or Palm with an onscreen keyboard (like the Palm TX, or any Pocket PC with a rectangular (not square) screen) - with those, you use the stylus to tap on letters, and while they're not quite as fast as a thumb-based keyboard, they make up for it by not requiring you to move your hands around when you switch between using the keyboard and the stylus. Thumbboards are most useful when you're using them to type several words at a time, as in an e-mail or a text message, but they're less useful when you're just entering a few letters.