personally I think its worth spending the money on Ortliebs, given my 25+ years of using them and their general robustness, specifically the mounting system, which is excellent and my original 1993 ones are still working fine (mounting system)
as per rollers vs packers, I use both and I woiuld say that the two advantages for the rollers is how you can put weird long things in them and clip the tops together , leaving space for said long thnig sticking out,
and two, the small strap and latch in the top middle of the pannier is handy for stickig your rain jacket under when rain is approaching, or an extra water bottle or whatever. Ive done this often on long trips when space was tight, like after a food store purchase of lots of stuff, or my rain jacket to save a few seconds before rain comes that you know is coming, or during one of those "on off" rain days, handy to just lean down and stick rolled up jacket under the strap.
I do believe howeever the packer fronts are larger, 30l vs 25 for rollers.
anyway, given the amount to spend more, I think its worith going with ortliebs for the long run.