I realize that 仏陀 is probably more common in writing where the reading is ambiguous anyway, but I am curious to know: do people tend to read it as ぶつだ or ブッダ? For what it's worth, my IME recognizes both, and the one time I did hear it in speech it was ブッダ.
Before you learn which is better, you have to know that furigara is not the same as pronunciation. Certainly furigana shows us how to read its kanji, but we sometimes don't write small hiragana or katakana because we didn't have such sounds before Kamakura Period. Hiragana and katakana are not so precise phonograms.
The answer is Buddha, ブッダ. I've never seen the form, ぶつだ, but it may be a sort of old way to write or miswritten.
Furthermore, most of people do not call him Buddha, but Hotoke(sama) or Oshaka(sama) instead. Sama is a suffix like title which used to call someone else out of respect.
https://furigana.info/w/仏陀 (based on old fiction): 60% of ぶっだ >< 30% of ぶつだ (plus weird people who want to spell ほとけ thus and similar).
kanshudo.com/searchq?q=仏陀 (based on modern use): 90% ぶっだ >< 10% ぶつだ.
Thus, it at least seems that ぶっだ is the more popular reading, and ぶつだ actually loses prominence.