The OP’s question is how much of the story he’d miss if he bought the two hardbacks. Short answer – none. Long answer – a lot.
The original Love & Rockets ran to 50 issues. Although it was created exclusively by Gilbert and Jamie Hernandez (with a few early contributions by their other brother Mario), it is an anthology comic. Most issues contain several stories from a handful of continuing strips. The two most important strips (both in terms of quality and depth as well as ultimately taking up the most pages) are Gilbert’s “Heartbreak Soup” (the Palomar stories) and Jamie’s set of stories about Maggie, Hopey, and the other characters who live in the Hoppers 13 neighborhood, aka the Locas stories, aka the Mechanics stories. (In the early installments the setting for the Locas stories was more sci-fi than it became, and Maggie was a pro-solar mechanic who could fix space ships and robots.)
After vol. 1 ended, Los Bros. worked on separate projects, some of which (“Luba,” “Penny Century”) continued characters from L&R. In 2001, they started L&R vol. 2, which is currently on issue #12.
The Palomar volume collects all the Heartbreak Soup stories, I believe thru the end of L&R vol. 1. The Locas volume is a little more complicated, because Jamie did some spin-off strips in L&R v.1 featuring Maggie & Hopey’s supporting characters such as Rena Titanon and Penny Century (that groovy chickadee). My understanding is that these don’t appear in Locas.
In addition to this spin-off stuff, I think the two hardbacks don’t include material from the solo projects or L&R vol. 2, much of which continues the stories of the major characters from L&R vol. 1.
But what you’re really missing is the rest of Love & Rockets. While the Heartbreak Soup and Hoppers 13 stories were generally the most powerful and took up the most room, there were lots of other strips in Love & Rockets such as Rocky Fumble, Somewhere in California, and one of my favorites, Errata Stigmata. These side pieces are often more sci-fi and tend to be funnier than the more well-known Palomar and Locas strips and IMO are an important piece of the work that Los Bros. created. This is stuff that you’ll only find in the original issues or in the TPB collections (which is how I read L&R).
Sorry this is long, but L&R has so much stuff and a long enough publishing history that it’s quite complicated. To sum up, if you buy Palomar and Locas you won’t miss any of the Heartbreak Soup story or the main Maggie & Hopey story thru L&R #50, but you’ll be missing some spin-off stories, later-written stories of the characters, and a lot of additional Love & Rockets material from other strips.
–Cliffy