The demodulator-level number is this one:
55481 total usable messages 55392 total usable messages
This is a count of valid messages that the demodulator found. I would have expected them to be identical in a perfect world but obviously something has drifted a little. That said, there is only a 0.1% difference here, which is down in the noise.
1.15 actually saw one extra aircraft in total.
The other main difference is in the CPR stats; these counters cover the interpretation of valid messages from the demodulator as aircraft positions. This area of the code has changed significantly in 1.15 to try to throw out a lot more bad data e.g. improbable aircraft movement, filtering out temporary drops in NUCp which tend to happen when an aircraft loses confidence in its GPS position briefly. So I guess that’s where your differences are coming from - 1.14 may be generating more positions, but some of them are probably suspect data.
You may want to retest with receiver location / receiver range provided as that affects how the CPR logic works.
Note that piaware takes the raw messages and does its own interpretation (in faup1090) which is based on the 1.15 code anyway, so all that really matters for piaware feeding purposes is the raw messages from the demodulator. The position decoding stuff only affects your local use of the data.