It is always helpful to hire an attorney who is experienced in the area of law where you need help. Whether a former prosecutor is any better than any other skilled criminal defense attorney is debatable, but depending upon how long counsel worked for the state's attorney's office, he or she may have significant experience in handling criminal cases.
The experience of the attorney is a factor to consider. Experience is a combination of prior positions, years in practice, teaching background, types of cases, bench trials and jury trials.
This answer/response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. Please do NOT use this answer/response to say or do anything regarding your situation. BEFORE you say or do anything consult with an experienced Federal and/or state criminal defense attorney in your jurisdiction who will listen to you and your concerns.
I'm not sure I understand the question.
A former prosecutor has experience prosecuting. That doesn't necessarily translate into a better criminal defense attorney though.
There are plenty of good defense attorney's that never prosecuted too.
You need a criminal defense attorney that you believe has the skills, experience, and attention to detail required. Are you comfortable with them? Have they discussed your case with you and gone over possible strategies and outcomes?
Aim for the best lawyer for your situation.
What you are essentially asking for is an opinion and I'm sure you will get different answers. There are many reasons why a defense attorney with prosecutorial experience is a good choice. While I would never say it's a necessity, it ceratinly never hurts. The extent of your lawyer's experience counts, but simply having been a prosecutor is only one factor (and the extent of that prosecutorial experience is relevant as well). You do want a lawyer familiar with the sysytem and the judges in any county in which you are charged. I generally urge people to consult with at least two experienced criminal defense attorney. Most of us provide free consultations.