I've owned the D800+ and the MG800 - both are reliable, quality amps with fantastic customer support.
I recently participated in a 3-way trade that meant I overlapped those amps for a day.
Set flat, the basic tone is different - the Mesa is a little darker and thicker, while the Magellan is a bit more clear and open sounding.
Both amps have flexible EQ and could probably be made to sound very similar, but they start at different places.
Since it's reasonable to assume that the 350-watt offerings share the same basic tone as their larger brothers, you can confidently pick whichever amp that sounds closer to the tone you want to end up with.
I recently participated in a 3-way trade that meant I overlapped those amps for a day.
Set flat, the basic tone is different - the Mesa is a little darker and thicker, while the Magellan is a bit more clear and open sounding.
Both amps have flexible EQ and could probably be made to sound very similar, but they start at different places.
Since it's reasonable to assume that the 350-watt offerings share the same basic tone as their larger brothers, you can confidently pick whichever amp that sounds closer to the tone you want to end up with.