I would second most of what is said above. You might want to try both, though the LH lounges are more of a hike at A/B versus the UA GFL in C/D. I think the food thing is also a bit subjective. More choice at LH, but I like the chilled shrimp at UA. Don't bother with any of the other UA lounges. However, neither holds a candle to the BA Concorde Room at IAD or UA's GFL at LHR