I agree with QuiltE if you home machine is a Janome it is much easier if your second machine is the same brand. I have the 6600 and also have the Janome 3160 as my second "traveling machine". All the feet and bobbins are interchangeable and the learning curve was almost non-existant. I had the Gem Gold and traded for the 3160 because of the needle up/down feature, could not drop feed dogs on gem, and the 3160 had a better feed system. Also the Gem is a 5mm machine and the 3160 is 7mm just like the 6600. They both weigh about 12 lbs are easy to transport. I just like sewing on the 3160 more than the Gem and it stitches beautifully.
Here is a link to the Janome America site that shows a comparison of the 3160 to the gems and 6600. Janome also lists the MSRP for each machine on its own webpage, but you can almost always find a sale, trade in, or deal.
Good luck in your search.
Murphy
https://www.janome.com/en/machines/sewing/3160qdc/