Player preference.
Most players using an Assassin Rogue build will likely suggest reserving the rogue's off-hand slot for a weapon, not a shield, on the theory that damage output is better than defense for the build. So on that note, getting the Red Leather Shield for Griss would serve you better. You're not likely to use the buckler that much and keeping Griss in a fight longer makes her more useful to your assassin build.
Counter point: Give that Griss gets to choose from a wide range of fire protection shields , including the easy to get Shield of the Forge and the top tier shield Shield of the Dragon Hunter, you might want to snag the Red Leather Buckler as the highest fire protection shield for the Rogue. If you need the fire protection, it's there for you.
Keep in mind that given the diminishing returns on elemental protection, getting above 110 resistance rarely alters real-feel combat abilities (presuming you still will do some healing anyways). Between rings, cloaks, skills, and potions, it's not to hard to stay in the 110 to 150 range against an element type.
A raw score of 110 fire resist gives 75% protection against fire damage. So if any enemy does 30 points of fire damage, that's reduced to about 8. A raw score of 150 (40 points higher), gives 85% protection, which reduces to about 5 points of fire damage. 3 points per hit might add up but in boss fights, you're likely going to use some healing potions, or doge getting hit anyways, so you likely won't see much of a game play difference. The difference is even less noticeable against enemies that hit with far less elemental damage (10 to 20 range). There are certain exploding enemies that having a high resistance might give you a little more room to breath if you can't outrun the blasts.
So while there are different tactics an difference theories, it really is up to you.