In the long run you will need both. EC/TDS/PPM pens are very useful. Find one on ebay for a decent price. As well as a pH Pen.

If your pH is off, you might show signs on nute burn when in fact its just fine. But if you are having problems u think are nutrients, maybe your pH is out of balance.

Best to save the money u would have bought on Litmus and just save for a Couple testers!

Good luck!