Laura,
I don't believe one is easier to get than the other because as you said, the criteria is the same. It might be easier if one is on the "border" of having enough work credits or on the border of having to much income in the family to qualify.
Every paycheck you recieve gets reported to the SSA because they deduct for medicare, social security... even if you are under the annual limit where you don't have to file taxes. So they do and should have a record of your work history and income. As for the times you were paid cash... you are out of luck there.
Based on your age when you file for disability (I don't know how old you are, if you want to PM me or post it here I can help you get a ballpark figure of whether or not you have enough work credits), you have to have a certain number of work credits. And the credits are calculated based on how much you earn each year, up to a maximum of 4 credits (I said quarter previously but I misspoke, it's per YEAR). It varies from year to year though. Ever year it increases as to how much you have to earn for it to equal a credit. Here's some information from the SSA website.
<a target=_blank class=ftalternatingbarlinklarge href="http://www.ssa.gov/dibplan/dqualify3.htm">http://www.ssa.gov/dibplan/dqualify3.htm</a> (cut and paste me)- The below info can be found at this website:
Before age 24--You may qualify if you have 6 credits earned in the 3-year period ending when your disability starts.
Age 24 to 31--You may qualify if you have credit for working half the time between age 21 and the time you become disabled. For example, if you become disabled at age 27, you would need credit for 3 years of work (12 credits) out of the past 6 years (between ages 21 and 27).
Age 31 or older--In general, you need to have the number of work credits shown in the chart below (chart is on the website above). Unless you are blind, you must have earned at least 20 of the credits in the 10 years immediately before you became disabled.
And here's the information on how to earn credits, <a target=_blank class=ftalternatingbarlinklarge href="http://www.ssa.gov/dibplan/dqualify2.htm">http://www.ssa.gov/dibplan/dqualify2.htm</a> (copy and paste me)
In 2005, for example, you earn one credit for each $920 of wages or self-employment income. When you've earned $3,680, you've earned your four credits for the year.
They shouldn't ask you much about your PAST income when applying for SSI or SSDI (unless they are determining retroactive benefits for SSI). They will ask you about your CURRENT income when applying for both, especially SSI. I always tell people, apply for BOTH because if you are inelgible for one (too much income for SSI or not enough work credits for SSDI) you can still qualify for the other. In other words, not qualifying for one does not automatically mean you don't qualify for the other... hope that makes sense.
So YESSSSSSSSS I would go into your local office and apply.
Hope this helps a bit. Let me know if you have any more questions, I'm happy to help.