I have both. The one you choose may depend more on your briefcase than anything else.
The beam patterns are extremely similar, but the X0 has a slightly brighter spill around the hotspot, while the P1 is whiter (the X0 is easier on your eyes, while the P1 seems bluish by comparison). The X0 has a larger bezel, of course, and is twice as long. The workmanship on the X0 is far superior to the Fenix P1. They are really both twisties, I find, because the Inova's push button is incredibly hard to push (the spring inside is larger and longer than it ought to be, IMHO). As far as light goes, they're pretty much the same, but the Fenix gets hotter, faster. It has less metal to act as a heat sink. And, as noted, the Inova will run far longer; if this is for emergency use, that's important.
As I said earlier, your decision may rest more on where you can fit -- and find -- the lights than on anything else, because if you're putting the light inside or attached to your briefcase, you want to be able to find it -- in the dark!
I have a small Fenix L0P with a spring clip attached to one of the shoulder strap D-rings on my Brenthaven computer bag, for exactly the purpose you're talking about. Inside, where it fits in a section of the bag made for pens, etc., I have an Inova T1 just in case I need something with throw. If you have a shoulder strap outside of the case, use the P1. I have my office keys, ID, cell phone, sunglasses case, and a whistle attached to my briefcase's D-rings with spring clips, with other stuff stashed inside. I'm not going to open my case outside my car, or go fishing around inside it when I need light. If you drop your keys in the dark, are you going to be able to open your case and find a light? And if you drop your keys and can't see them without a flashlight, having a flashlight on your key ring is silly...
Of course, inside the car are three lights (Inovas for flood or spot) and 6 spare CR123 batteries.
But on my desk at home is a SureFire U2, an Inova T3, an Inova X0 (reflector), and an Inova X5 (newest version), while the Fenix P1 and a Jil CR2 near my keys, with spring clips to attach them to things. The one I seem to reach for when going outside the house for something is the new X0 (but as I do so I curse that spring).