I've seen all of the ARC models and some have the moveable arms, that don't really help arm strength much, but do enable you to easier get up to a full sprint. The ARC's without the arms, make it harder to get up to speed quickly and really burn the thighs! You can also use the "armed" version and simply not accentuate the arms as much. I can see why Valby likes the "armed" version with the intensity and speed someone like her would use. The various numbered models are similar, besides the having or not having the arms, but have a more complex display board with more settings as you get into the 700's versions. My condo complex has a 425a ARC, which is smaller for an apartment with less space available. If I were to be looking for a used model for home use, I'd go for a 625a, which has everything you would need. ( I have relatives who recently purchased a 625a for use by their son, a 48.6 quarter miler, who tore his ACL during basketball, and will use the ARC as part of his recovery. The hardest part? Getting this very heavy quality piece of equipment into their garage gym. New models are $6000-$10,000 and they got a good used 625a for $1500. (Hint: If you use the arms on the ARC, grab the arms in a low position. If you grab high, it feels awkward and unnatural. Low or mid is best!)