Before I started to sew jersey and other knit fabrics, my main thoughts were about the stitch quality. Now not only is the stitch important, but also the quality of the knife/blade and it’s cutting ability, which you only really find out by trying. My Husqvarna which is 25 years old, with brand new blades, can’t cope with silky jersey where as my nearly 40 year old, 3 thread Frister Rossmann, cuts through it like a knife through butter. Also make sure it comes with a spare knife or purchase a spare, as apparently these are the parts that become obsolete quickly. I had to send to the States for my Huskylock blades.
My advice would be to try as many as you can. If the shop offers the machine you want, but not the guarantee SMD is offering, tell the shop and see if they will match them. If not go to SMD.
I’ve noticed that most suppliers offer the same deals and prices, especially on the popular machines, and most say if you find cheaper elsewhere they will match.