I believe you can tweak the algorithm to suit your requirements, e.g.

    openssl pkcs8 -topk8 -v2 aes-128-cbc -in ~/.ssh/id_rsa -out test_rsa_pkcs8

As far as the strength of AES vs. 3DES, the difference is probably negligible for most practical purposes. AES is however the newer standard, and is more flexible, performs better on different hardware etc, so seems like an overall better choice, but I would say not so much for any practical security properties. (i.e. both are pretty strong).