It is almost impossible for someone to say if it is a good deal or not without a full review of the police report and the case. Yes, a 602 charge will be better for your immigration than a petty theft charge. There are a lot of advantages to the deal but you should hire an attorney to review the case prior to making any deals.
As a general answer it does not sound like a bad deal. Sounds like you already plead to the reduced charge. If so, your best bet is to complete probation satisfactory, including not picking up any new cases. Upon satisfactory completion of probation you can ask the court to expunge your record, pursuant to Penal Code sec. 1203.4. The court charges a fee of $120.00 to expunge.
For some purposes, including immigration, this conviction will stay on your record. However, I do not think that PC 602(n) is a crime of moral turpitude that will preclude you becoming a citizen. You will need to consult some one specializing in immigration law.
without knowing the amount of the alleged theft and other factors, it is hard to say. however, PC 484 is a crime of moral turpitude and is definitely BAD for jobs and immigration purposes. therefore, a 602 is always better than a petty theft as it is NOT a crime of moral turpitude.
once you complete your probation, you can expunge the conviction from your record pursuant to penal code section 1203.4. HOWEVER, you will still have to report the conviction to the ICE or other governmental agencies. it will allow you to state on most job applications that you were not convicted in this case. the only way to remove it from your record it to do a Motion to Seal the arrest. this requires a finding of factual innocence so would not be granted. you would also have to reveal it if applying to be licensed (car salesman, real estate agent, attorney, etc) but you would put down that you had the conviction expunged, so always do it as soon as your probation is complete.
THIS IS A GENERAL OPINION ON THE LAW AND DOES NOT REPRESENT SPECIFIC LEGAL ADVICE.