How do RSA and ElGamal key sizes compare?
RSA and ElGammal are about equally secure at the same modulus size (assuming, of course, intelligent parameter selection in both cases).

For RSA (assuming you use good padding), the best known-attack is to factor the modulus with NFS.

For ElGammal (assuming you use a subgroup with a large enough prime factor), the best known-attack is to compute the discrete log of the public key with NFS.

Now, the version of NFS to compute discrete logs is somewhat more complicated than if it were used to factor; however I don't believe that extra complication adds a significant time to the time taken.