It would be helpful to know how much stringing you plan to do but I think at the end of the day Irwin is correct--it really comes down to which clamping system you prefer--other than that, these two machines are substantially identical--I think the mounting system on the 1500 is a bit different but not necessarily better. I have strung on all sorts of machines and Irwin really does identify the differences in clamping systems. My personal preference is glide bars for really two reasons:
1-simplicity in design/easy to maintain: there are minimal things that can go wrong with the Neos 1000 glide bar system--keep the bars and the clamps clean and you should be good for a long time. There's virtually nothing that can happen to the bars themselves and if you need ever need a new clamp, they are reasonably priced and obviously no hassle to install. I have always been concerned that the more complex swivel clamp just means there's more to go wrong and more to fix when it does.
2-speed--glide bars are super fast--even changing them out for the crosses results in almost no time lost.
That stated, swivel clamps can be fast too when you get used to them and they do provide more options on fan and other non-traditional string patterns. If you plan to do lots of varied-pattern racquets (and I almost never see a fan pattern except on the racquetball or squash courts), particularly for profit, I might lean to the 1500. Personally, I no longer string for anyone other than myself (and family, and occasionally some hitting partners)--and I actually use a mint condition (always owned by me) Ektelon H (the father of Neos). Based on my needs, if I were to buy another machine it would be the Neos 1000--but if I thought I would be stringing a great many and varied-pattern frames, I'd go with the 1500.
I will concede the 1500 looks cooler. The good news is in either case you will be getting a good machine and the good folks at Tennis Machines can assist you with any problem you may encounter. Good luck.
P.S.: also pick up a good starting clamp.