I just got an Old Man Mountain Cold Springs rack a couple of weeks ago. Before that, the rack I had on the front was platform only, so I've only been playing around with front panniers on my bike for a couple of weeks, but I have always liked having platform up front. It may be empty 90% of the time, but that's part of its utility. When the odd item pops up and you would have to do a lot juggling to fit it on the bike, instead you just strap it to front platform. Yesterday my wife and I took a short ride in a city we don't know well. I made a little cue sheet of our route and taped it to the front rack platform. Other odds and ends were in panniers also on the front. On future tours and overnight trips, I hope to get most of my gear in panniers, with minimal use of the tops of either of my racks, but I still like having the platforms there, just in case.

A front rack with a platform has more metal and, therefore, more weight. Whether it's enough weight to concern you is a personal preference. I just love having the front platform even if I don't use it often. I would say start with a platform/pannier rack and if you don't like the weight or don't use the rack, consider switching to low-riders. Of course you could just as easily start with lowriders and see if you find yourself short of storage and needing a platform rack (and I think there are some low-load capacity platform racks that mount independently of the low-riders). I guess the question is would you rather be potentially over on weight or under on storage. I always opt for more storage, but then I know that a weight penalty comes with that.