I'm a native speaker and I would use the first sentence rather than the second. The difference is that the first can be literally translated as "I have seen a blister (with pus) in my chest." while the second can be translated as "I saw the blister (with pus) in my chest." Typically the first expresses either a possibility of seeing (may nakita means "to have seen") while the other usually is used to state a fact that "I have seen." Also the first usually is used to express "I have spotted" while the second is used to say "I saw." Hope this helps. PS pagsasalamat is by convention shortened to pasasalamat.