By Arc, I mostly mean the old Arcs since I don't have a new one yet.
Arcs were machined in the same factory that now makes Peaks. In fact Peak got started after Arc and Arc's contract machine shop had a disagreement that I won't rehash and they decided to go their separate ways. Arc (the old Arc) shut down shortly afterwards, and the machine shop's management decided to stay in the light business so they started Peak. The quality of the metal parts has seemed to me to be just about the same between (old) Arc and Peak, which is what you'd expect.
Arc has always done a better job with electronics. Peak started out not so hot and has improved some since then, but the latest Arcs are another step ahead.
As for the two companies, well, Arc LLC had its foibles and messed up in a number of areas, but we can surely say that its owner is an actual flashaholic and has always pursued a flashaholic vision in flashlight design. Arc has also been communicative with CPF about technical issues--we know the make, part number, color and flux bins of all the LED's that went into Arcs, while we don't even know who manufactures the leds in Peak lights. So Arcs have always been popular here, even through Arc LLC's mostly-self-inflicted business troubles.
Peak seems to me to be, well, just another company trying to run a profitable business and keep customers happy (and doing a good job), but which, deep down, hasn't been that flashlight centered. Their products are well made and sometimes innovative (double ended CR123 light) but have usually struck me as missing the point in some way. It's as if, not having much of a vision of their own, they made a bunch of different stuff sort of at random, to find out what people would buy. The Peak products that have been most successful (Matterhorn) are copies of Arc products, and what's more, have been through several design iterations and have improved basically by becoming closer and closer Arc copies.
Peak has, however, been willing to try new things (the "snow" leds and their 3-led AAA lights have in particular become popular) while Arc sticks to its guns and declines those ideas based on particular flashaholic technical criteria that not every flashaholic subscribes to. Peak has also had the resources to crank out a far wider range of products, many of them not too successful. Peak has also done a better job keeping its manufacturing program in order. Arc LLC was constantly beset by production problems of various kinds that eventually led to its demise, and the new Arc may be making similar errors (you can't place an order for a new Arc right now). Peak's new product introductions haven't always been on schedule (that's entirely normal) but once introduced, they've generally been able to fill demand.
All in all, I'd say Arc has been more of a leader that sometimes stumbles, while Peak has been more of a follower that's stayed steady. I like the products Arc has been making so I hope the new incarnation of the company keeps its act together and stays around.