My wife's got a 2011 Mamba as her first boat. I'll say that it is super forgiving. Sometimes I wonder how she didn't go over (floating sideways thru class 3 wave trains, etc...). It's got a semi planing hull (so youll be able to learn the basics of surfing fairly well in it, and edge control) and softish edges. Plus, it'll work for a good creeker as you progress, so you shouldn't really grow out of it. You'll be able to keep it long term.
I've heard similar things about the diesel, but have not had the pleasure of trying one out. From what I've heard, I'd expect it to have slightly harder rails.
Being new, like has been said, you won't have a bias for any certain design. So either will likely feel right to you after you get some seat time. So I'd get whichever is cheaper/better feeling.
And if the sale prices are anything like my kayak shop this past winter ($500-600 for a brand new boat), I'd snatch one up for sure even though you're a beginner, and the boat will likely take a beating as that's pretty mch used pricing.
Good luck, and see you on the water.