katsparrow

I'd considered the the Skyline S6 and Juki dx7 too, similar to the Brother NQ900. If your space is limited, those are all great choices, and would be perfect for most sewing! If you try each one out maybe you'd find a preference- otherwise I'd go for the best deal and whether there's a local dealer to service the brand you choose.

What I love so much about the Juki straight stitch machine is that it's so powerful, smooth, simple, and has beautiful stitch quality. The Brother NQ900 has nice stitch quality and sews very smoothly and quietly. It's reasonably powerful, but not to the level of the Juki TL2010Q. That Juki is a workhorse since it's semi-industrial so can easily handle bags and thick layers- whereas the NQ900 can handle some but I don't want to push it. If you're making bags with your current small Brother, I'd expect the NQ900 would be better- but might not be enough power for a serious bag maker, or layers of denim or canvas. Leather- I'd reserve for the Juki. If you're not sewing a lot of heavy duty bag materials etc. then the NQ900, or the similar ones you listed, are probably fine.

Another feature I really love about the Juki straight stitch is that the feed dogs aren't super wide and the straight stitch plate is always there. That helps reduce the feed dogs pulling the fabric down, such as at the corners of quilt blocks, or when sewing lightweight fabrics like fine cotton lawn or voile. My NQ900 has a totally separate optional straight stitch plate but it's a pain to switch it out with the standard plate. Some multi-stitch machines have a new easy converter switch to turn the regular wide hole plate to a single hole straight stitch plate, without needing to change the plate at all. Maybe the Juki dx7 has that button/lever? It's a really nice feature. But it's not an issue for the Juki straight stitch machine, since it's a constant straight stitch, and fabric glides over the narrow feed dogs without getting sucked down nearly so much- another reason why quilters and garment sewers love it. I still may use a "spider" to ease the beginning and ending of stitching on the Juki straight stitch, but it needs the spider less than machines with the wide hole plate/wide feed dogs.

Also, I mentioned how I love being able to see so well with the Juki- it makes a huge difference to me to have better visual clearance while sewing, not having to duck my head to see the fabric under the machine. I learned to sew on my mom's old Viking which is a similar sleek upright shape as the Juki, with a high shank- it's just a lot easier to see things. Since I started to sew again, on modern low shank machines, I'm bugged by not having the same visual clearance. But it's probably not a big deal if you're used to the low shank bulky machines like NQ900.

About the thread tension- the Juki requires setting the thread tension manually (I think all straight stitch machines do), so there's a bit of a learning curve if you're not used to that- but you get the hang of it. Yes, the auto-tension on the Brother NQ900 and similar multi-stitch machines is super nice and easy, so that is a good feature. Any of the multi-stitch machines you listed should be great, just don't go too heavy duty with the bags. Perhaps there's a Juki TL-2010Q straight stitch machine in your future somewhere, when you have more space- lol! That's a lot of info I just shared- well, don't let it overwhelm you, it's stuff I wish I'd known before, so I hope it helps as you sort out your choices!