The short answer is both. Absolutely climb both if you have the budget, and more importantly, the time to do so.
Lion Rock is notoriously expensive (tickets cost $30USD) and some backpackers tend to not climb it to avoid the entrance cost. Nearby Pidurangala Rock offers a good alternative at a fraction of the cost. It also has the benefit of being able to see Lion Rock in from the top.
Both offer incredible views overlooking the area. There was something really special about climbing up the Lion Rock. An ancient fortress built on top of a giant rock 1,500 years ago. It reminds us quite a bit of Machu Picchu.
So, to summarise the slightly longer answer:
Lion Rock: Prioritise Lion Rock if you only have time for one
Pidurangala Rock: Visit this one if you don’t want to pay the $30USD entry fee but still want phenomenal views