They are indeed both link state but there are some differences.
OSPF works actually with 6 types of LSA (Link state updates) and multiple area types (5 if I remember correctly)
The IS-IS is based on CLNS and uses only 3 types of LSA's (LSP called in ISIS) which makes the ISIS A bit 'easier'.
It only uses 3 levels (level 1,2 and 1/2.. the other levels previously used are obsolete by now)
Also the AD is different (115 for ISIS and 110 for OSPF)
Also the levels in ISIS can contain upto a 1000 !! routers where OSPF should be capped up to 50.
Just play a bit with ISIS and see for yourself which you prefer. OSPF is still widely the most used protocol I believe (over EIGRP).