I think im travelling however compared to some on this site probably looks like a weekend away. 4 weeks away round europe with the wife looking between 3-4 thousand miles !!!?
one big single trip and multiple overnight hotel stops will mean you'll either want quick release cases or bag liners that pull out quickly.........or soft throwovers that release together in one big lump.
Hard cases seem better for bigger trips, soft panniers for long weekends away - with a big crossover area in between depending on circumstances
http://www.andystrapz.com/products.php?PCID=51

