No matter what sclerosant is used (hypertonic saline, sodium tetradecyl sulfate, polidocenol, or sodium morrhuate are the most common ones), it needs to get inside the spider vein to do its work! The smaller the spider vein, the harder it is to hit properly. Sclerotherapy, as you have found out, is highly operator-dependent, and less related to the solution used. All of the commercially-available sclerosants work well, but have different side effects, proper concentrations for uniform response, and different adherents among users. But all of them have one common denominator--they have to be skillfully and properly gotten into the vessel for it to "work." You have had poor results, I would submit, not because you have had a "wrong" solution (though to be truthful, it can be a partial factor), but because your injector simply missed the veins too often.
As someone who has performed sclerotherapy for 30 years, and taught laser surgery for over twenty, I can still unequivocally state that skillfully-performed sclerotherapy remains the "gold standard" for treatment of spider veins. Larger varicosities require other treatments, but it's the smaller ones that require the best technique and often are the biggest cosmetic concern. Lasers can work for some of the smallest spider veins, but usually at slightly higher risk of complications (blister, scar, pigmentation change, or inadequate result), and almost always at higher cost (It's the laser!).
Find your old sclerotherapist, or a new one with better skills. BTW, since you asked, I believe polidocanol is the best sclerosant, as it is the most pain-free, has the fewest side effects, and has great results when used in 1% concentrations with proper technique. I have use all of the others, so this is based on significant experience with these.