At the same age, Forster by some margin I'd think and that is the only way to compare them. Remember that Forster has played at a higher level for much of his career as well. If they were sold tomorrow, with Murray's injury record, I'd say Forster would sell for more too.
That Sky Sports interview posted on here would suggest his aim is to run out with the team at Falmer, quite a stretch to expect him to still be in the 1st team in two years, especially if his goals fire up up the league. I expect him to be still here but not convinced he'll still be a regular...