...probably one has phono(8200) and the other one hasn't.
The 8150 was offered from May 1988 through July 2000. It was replaced by the 8200 MKI in July 2000, which was subsequently replaced by the MKII in October 2001.
Both the 8150 and 8200 MKI use the same phono stage which supports both MM and MC, but a component change is required when changing moving magnet to moving coil.
The 8150 can be upgraded to a 8200 Mk1, which offers improved power regulation to phono and line amplifier stages. Also, the 8150 is rated at 150W into 8 ohms, while the 8200 is rated at 175W into 8 ohms, 350 into 4 ohms.
The 8200 MkII has a different, FET based phono stage (no component change required when changing moving magnet to moving coil), and offers increased current capability compared to the MKI.
Unfortunately, the Plinus web site states the MKI cannot be upgraded to a MKII (which I had hoped...I own a MKI).
I have heard from other Plinius owners that the difference between 8150 and 8200MKI is substantial, as is the difference between 8200 MKI and MKII. Primary improvements from model to model include tighter bass resolution and presentation, more liquid midrange, slight improvements to high end.
While I have been told by one source that the 8150/MKI phono section is essentially the guts of the Plinius Jarrah external phono stage, I have been told by others that well-regarded external phono stages (i.e., Blue Circle, EAR) provide better analog presentation than the 8150 or MKI, which has been described in A/B comparisons as muddy and slow (relatively speaking). I personally have not A/B'd vs. standalone phono stages, just reporting what I have read and been told.
Hope this helps your research. See if you can demo an 8200 MKII. I am told it is competitive with similar units at much higher price tiers. I am driving Maggie 3.5's with my 8200 MKI, listening mostly to analog, and love the music. Perhaps the upgrade path just hasn't bitten me too hard yet.
While I cannot compare these models from personal experience, I can say that my 8200 mkII (bought $1800 used here on Audiogon) is very pleasing indeed. I'm a new audiophile so it's hard to explain quite why, but I just love the sound that comes out of my system now. I highly recommend trying one out (give it a week with power on before evaluating seriously).
Now I'm thinking of getting my hands on a MKII to compare. A bit more information for anyone interested.
Authoritative sources tell me that the power supply in the MKII is substantially larger than the MKI - thus able to deliver far more clean current into my amp-hungry Maggies during dynamic demands. Also, the phono section in the MKII actually uses less parts than the MKI for cleaner & shorter signal path.
All together, the MKII probably really deserved an entire new model number, as the MKII moniker doesn't do the full improvement justice.
If I can compare the two units, I'll post more detail.