Go the aluminium box route. I had the BMW ones and was disappointed. They look smart but the side opening made the use of inners essential and still hard to use all capacity. Sod to get in them in a crowded bike park, small capacity and when expanded the limited attachment to bike eans they move around.
Changed to Touratech and they are great. The frames are are bit obtrusive compared with the BMW ones but I would rather have something that does the job than something that looks good but doesn't.
The panniers are also a bit agricultural looking but gives the Long Way Round/Down image - but more important is they work. The top opening means all capacity can be used, the simpe rectangular shape is also good for using capacity. For big stuff you can just leave the lids off.
Only downside is the bare aluminium marks easily. Metal Mule and others powder coat there versions but that costs a lot more and after using them for a while I prefer the loose lids on Touratech.
