If your lights were dimming, it was your power that was bad - could have been due to the cable, or the cable just added to problem.... if you have a fridge, freezer, or other things in your garage (or on that breaker) that draw... adding the trailer was likely too much. Our garage has the outlet by our entry, the outside lights, and the garage on it, for example.
Having said that -- The 30AMP cord is the way to go. I've seen adapters melt into a pile of goo, then trip the breaker. And the heat will get worse as the power does.... this particular example was at a cool fall contest -- the power was marginal to begin with; but as everyone was firing up thier lighting, electric heat, etc. it overloaded the box, and had everyone running hot due to the poor draw. guys with cheap cables had them ruined.
If you think you are going to need a long run, you want more cable than you need anyway.... if you pull less amperage than you need and get into a low voltage scenario -- you can ruin the capacators on your fans/compressors, etc.
I even went one further - and ran 50AMP RV cable out of my Concession trailer. Todd has two 30AMP power supplies in his. EDIT: just saw Todd's previous post. The two 30 AMP setup is really slick!
Spend a little now, save alot later