I taught Secondary for ten or so years, including 2 A levels and i've taught year 6 for the last ten years. Primary is harder in terms of marking load, the pressures for results just as heavy, if not more so. Behaviour is easier in some respects, but not all. The high end SEND have gone by secondary, but you are expected to teach primary classes with very high levels of ebd and SEND in. I've been spat at, kicked,bitten and scratched by far more year 6 students than I ever was in secondary. There are fewer TA's than ever, very few echp and practically no send school primary places.
I miss the teaching of A level, but keep my hand in by exam marking it. I left secondary when teaching to the test became practically mandatory and students who were patently no A level calibre forced to stay on. Primary is now no better - the sats papers are extremely proscribed and if you don't teach to the test you won't get the ridiculous targets you will undoubtedly have been set.
I saw far less of parents in secondary. You see a lot in primary. I also deal with all the social service meetings for any child in my class, liasewith family workers etc. This was the pastoral hoy job in secondary, not just the class teacher.
I take a lot more work home in primary. Every book marked every evening. The planning is more intense with no schemes of work. In secondary I could use my sow and plan as I went. My weekly plans in primary must be tailored for "old" levels ranging from p scale to 8. My class sizes are also larger - 36 now. My gcse and a level classes were never that large, although they may be now.
For all that, I wouldn't change back again - largely because as an older, menopausal teacher, i don't think teens would have any respect. For my year 6 I am verging on grandma years so they still have some! Plus i'm just too damn old and expensive for just about any school now.