Phenolphthalein > Thymolphthalein
Phenolphthaleinis an every purpose - indicator and has also been used in schools. Unfortunately 2009 it has got a harmonised classification and is now carcinogenic and probably mutagenic and toxic to reproduction. In fact only very little of the indicator is needed to prepare a suitable solution and it may be that the final concentration is less then 1 % and therefore the solution is no more carcinogenic concerning hazard regulations. But there must be anybody who has to prepare this solution!
Those who do not need a color change from colorless to red but can accept also a color change from colorless to blue may use thymolphthalein as an alternative which is not classified to be a hazardous compound. The color change is on a similar pH-range (9,4 - 10,6 instead of 8,2 - 10,0) and since only very small amounts are needed it will be of no consequence that the price is 3 times higher. Thymolphthalein at the moment is not classified to be carcinogenic. It is likely that this fact will not be changed in the future because it is well known that alkyl side chains - which make the difference between phenolphthalein and thymolphthalein - are pushing back the carcinogenic activity.