Lion Crashes MBP with low GB capacity?

I bought a 13'' MBP 2 years ago, it had 150GB when new, and when I upgraded to Lion everything seemed perfect, 4 months after I upgraded (3 weeks ago) my mac started crashing constantly until it froze in the initial white screen with the apple logo and would not go on.
I had to take it for a reset and now I have snow leopard again, as before. The guy that ''fixed'' it said it was because Lion needed much more capacity than 100gb and it slowed my mac down...he was not an apple technician since there's not a genius bar in my town, so I don't know if it's true. It doesn't seem right that I won't be able to upgrade to future OS X...
Is it true? Should I install it now that I have full 150GB and buy an external hard drive?
Thanks for your help!

Steve's Barber said:

Did you ask this "genius" why your mac ran fine for all those months with Lion on it?

Your drive was probably filling up - nothing to do with Lion as Snow Leopard needs some free space as well.

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Great! I guess I'll intsall it then...I really liked it!

MisterMe said:

Your boot partition should have no less than 10% of its capacity available as free space. For 100 GB of a hard drive to be an issue, it would have to be a 1 TB drive. The technician was full of it.

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He WAS full of it! I was getting worried! do you have any idea why it could've crashed? It happened two weeks before and my mac started up after three hours, so I was able to save some of my files before it happened again, and this time it didn't start up at all, just froze in the white screen...