Company makes it mandatory to go through their contracted attorneys office for PERM application then I have the choice whether to continue with that attorney for I140 & I485 or choose an independent attorney. Either way I'm paying the fees for both the applications. Any pros or cons on this issue such as delays, etc.

As long as the attorney is competent and will keep you updated on the case, either will be fine. Please note that if you hire an attorney, they will certainly speak to you more readily. There is an element of dual representation, so if you use the company attorney you should be treated fairly. Their first loyalty will still be to the company.

Without knowing any more, I would find out how many cases they handle for the company as that is a good indicator or overall competence. If you are not comfortable with their experience, find your own; after all, you are paying for the work.

Thanks NYcity boy, my company is a nationwide one listed on the stock exchange so they have a lot of sponsored workers. The attorney I guess is a good company since they get all of that work and they have law offices in about almost every other state. My main issue is that if I get my private attorney is there anything different he/she might be able to contribute in terms of speeding up the process.
I've got no one that I know that went through this process would you have any recommendations or sources I can refer to find one in California?

All good. Most large companies use one of the large "national: law firms, but you should find out which attorney at the firm will be working your case, then you can check on their credentials. Nothing that any other attorney can do in terms of speeding up the case.

One thing to consider is that the firm used by the company will have all the company information and contacts set up to work the case faster. The downside is that most large firms push this work to a junior associate.

You should be fine going with the attorney used by the company. If not, check for a referral from AILA, the American Immigration Lawyers Association. Good luck!