I have owned the Moon I-5 for about 9 months. In my room, it can sometimes be a bit lean. I have heard this from other people on this forum too. However, since changing to warmer interconnects and speaker cables, and adding a Bel Canto DAC 1.1, the Simaudio is now having a chance to show it's true strengths. The I-5 is very good at creating a precise and large soundstage, as well as a convincing and detailed portrayal of voices and instruments. Very, very lifelike.
The choice really depends on your tastes in music and system balance, and what kind of acoustic characteristics your room presents. That being said, I have often read on Audiogon that the Plinius in general is a warmer sounding amp than the Simaudio. And if you're comparing the Plinius 8100 to the I-5, I think the power difference is more like 100 watts versus 70 for the Simaudio. The Plinius 8200 is rated at 175. The I-5 sounds much bigger than 70 watts with my Sonus Faber Concertinos and ProAc Tablette 50 Signatures.
I have heard the Plinius amps but not integrateds only in a store, and they sounded fantastic. I think the choice is dependant on your taste and associated equipment.