Ravinder,

If you are using both Immersion silver and OSP, I believe that the Immersion Silver will be the replacement for the HASL, OSP can be applied selectively (not the entire board).  But neither provide good long term corrosion protection over traces.  If you must route high speed traces on outer layers, consider adjusting the trace width of the high speed traces to account for a solder mask cover.

You may want to subscribe to the Designer's council listserver for more advice on PCB design questions.

Pete