My thought is that a "Spare Air" unit is too small to be really useful. A "real" pony bottle (12, 19, or 30 cu ft) and low end regulator are about the same cost as a "Spare Air" and provide 4 to 10 times the air reserve as the "large" (3 cu ft) "Spare Air" and about 7 to 15 times the air of the small (1.7 cu ft) "Spare Air" unit.
That said, I have 2 observations regarding your situation.
1. This would have been a good question to ask before you bought the "Spare Air"
2. Since you already have a "Spare Air" you are better off with it than without it. (i.e. better to have a little reserve than none)
Let’s compare ponies. Assume a "stressed" SAC rate of 1.0 cu ft / min (after all you did just run out of air, so you are probably a bit stressed).
Spare Air 1.7 - 1 min at 15 ft (short safety stop)
Spare Air 3.0 - 1 min at 30 ft or 2 min at 15 ft.
AL 19 - 30 ft/min ascent from 120 ft to 30 ft (3 min) and 1 min stop at 30 ft, and 5 min safety stop at 15 ft. (light deco)
AL 30 - 30 ft/min ascent from 130 ft to 100 ft, and 1 min deco at 100 ft, and 30 ft/min ascent to 70 ft, and 1 min deco stop at 70 ft, and 1 min at 60 ft and 1 min at 50 ft, and 1 min at 40 ft, and 1 min at 30 ft, and up to 3 min at 15 ft. (adequate deco)