What would a new machine have that your current machine is lacking that would help you improve your skills? Variable speed, needle down, knee lift, larger harp/throat etc. Things like these are nice but would they improve your skills? After analyzing this I'd take some practice pieces of fabric and thread and go test drive some different machines. Take a sharpie and write on your test pieces what machine you used and then compare the different samples. Don't forget to make on using your current machine. Sometimes it's more a matter of wanting a new machine rather than needing one and that's OK too.