Broadly speaking, modern espresso groupheads for home use you want to focus on one thing: responsiveness. You want it to react to temperature provided, not resi

Broadly speaking, modern espresso groupheads for home use you want to focus on one thing: responsiveness. You want it to react to temperature provided, not resist changes to temperature, and be able to be controlled. E61 groupheads are typically coupled directly to the boiler of the machine via thermosiphon, so they heat up as the machine heats up. This also means that heating up the giant chunk of mass takes ages. Terrible for home use. The E61 grouphead is meant for commercial usage. It's heavy, tons of thermal mass, and has a tendency to overheat if it isn't flushed regularly (or, in the case of a commercial setting, just used... which typically isn't a problem in a commercial setting). For most home usages of E61 machines you need to perform "temperature surfing" to get a great shot from an E61. More modern groupheads don't have such a large thermal mass and tend to be somewhat isolated from the boiler or thermoblock, so you can just heat them directly from a previously pulled shot and be just fine. Again, not to say you can't make good espresso from them. I think there's also some E61 constructions and mitigate some of these issues, but they're not standard in the industry.