Is this a trick question? Because there's no wrong answer here. That said, I'd probably choose Nepal over Peru.
From my point of view, Nepal is a bit more exotic with the world's highest mountains, the eclectic catch-all city of Kathmandu, various religious expressions, and indigenous cultures that are far removed from our Western way of thinking.
Plus I think Nepal is a bit safer in remote areas. On the other hand, Peru has tremendous Mayan ruins, endless green terraced hills stretching as far as the eye can see, a diverse cultural history, and great wool sweaters.
Before you choose, I'd take a serious look at the cost discrepancy between the two countries. From the United States, it would probably be much less expensive to go to Peru, but from England, it's a toss-up.
I did a land-only price comparison with KE Adventure Travel, a company that offers trips to both Peru and Nepal. Surprisingly, at $3,295, their 21-day trip called "Peru's Remote Cordillera Vilcabamba" was more expensive than any of their 21-day Nepal trips, which ranged in price from $2,275 to $3,195.
Ironically, the picture they show of the trek in Peru looks as if it was shot in Nepal. If you'd like to use an outfitter for this trip, contact U.S.-based KE Adventure Travel at 303-292-0458, or e-mail them at firstname.lastname@example.org.
I'd also try to get a sense of the safety situation in each country. You may be a person who likes traveling among guerrillas, but if you don't want to hassle with political uprisings or a possible kidnapping, you should try to keep on top of the situation in Peru.
A good way to do this is to get in touch with the U.S. State Department. Though Peru is much safer than it was a few years ago, there are still isolated incidents that will make your blood curdle.
Last but not least, I'd spend a few hours in a book store browsing through the picture-book section trying to get a feel for what you'd actually see in each country. Then I'd take a deep breath and go with your gut.