You'll *probably* get more life out of a genuine Mitsu, but not necessarily, and maybe not as much more proportionally compared to what you pay more. I got two third party lamps/housing recently for the cost of one Mitsu on Amazon, and because I have Amazon Prime (free shipping), it worked out to less than I would have paid for one genuine Mitsu.
Really comes down to what you're comfortable with. I decided I'd rather have two new lamps (one as my emergency spare) this time. I also have two of these TV's (a 65" and a 60") so that extra new lamp is now the spare for both of them. But for years before that (my first Mitsu DLP was a 2004), I always bought genuine Mitsu.