Behind the ear hearing aids are evolving into software driven devices.
The hardware will be the same across the model family but the different
levels of features (and pricing) will be determined by the software.
Hearing aids will be upgradeable with respect to tweaks to the OS but
probably not the feature set except in minor ways.
The current push is to use small receiver-in-the-ear aids for severe
loses by using custom earmolds and larger, stronger receivers and more
aggressive and effective anti-feedback algorithms.
Bluetooth and wireless connectivity will allow aids to run in sync with
each other and allow direct connection with cell phones and TVs and
music players.
These days, it's likely that you'll be steered towards getting a behind
the ear hearing aid when making a buying decision. This trend is likely
to increase as the fitters get more comfortable with fitting severe
loses with BTEs.