I have sewn on a lot of machines, anything from a singer 111w, phaffs, consew 226r (my personal machine for 30 years) consew 206r, 206rbl, 744, marrow machines, sergers. and computer controlled embroidery machines. I'm restoring a singer 144w 30" long arm, with no reverse. I personally like consews but any machine you feel comfortable at and sews without skipping stiches or jamming is a good machine. a good retailer will let you try any of their machines. I heard one local business that will let you try a machine for a week. try as many as you can, you will find one that is rite for you.
Guess I spoiled myself. I've been in a few shops and most machines I see are as old as 70 years. The day I turned from hobbiest to business owner, I powered up a brand new servo driven Juki DNU-1541. So easy to use, very smooth operation and the tensioning system keeps a steady strain with no drift at all. So far, i guess I'm a juki man, looking at thier LU-1510N Top Stitcher
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I'm using a Mitsubishi LU2 4710. It's really more of a factory or production sewing machine, but man this thing is awesome. Servo motor with needle position sensor, auto back-tack, auto thread cutter, auto foot lift, adjustable walking height, etc! I bought it used for around 2K. the new ones are really expensive.
If your are looking to replace your main machine, I would definitely look into a servo computer controlled machine. Though maybe not the Mitsu. its an awesome machine but the manuals are poorly translated, parts are pricey and a bit harder to find. There are plenty of manufactures making these machines. Although you will pay for the quality of a production machine, they are made to run all day year round. In my 7 years with my machine the only part I've ever replaced is my feed dog, and it was probably a 10 year old machine when I bought it.
For an additional machine the double needle long arm would be an awesome addition. I would love to ad one of those to my arsenal as well as a double needle post bed for french stitching. Personalty I would go for the post bed first. Though the flat bed would have cut my time in half sewing a full yard of 2.5" double stitch diamonds yesterday.
I'f you have been thinking about the double for a while now I would go for it. You probibly find yourself saying the same thing as Naseem