Sorry for coming in on this a bit late! (New member.)
In my collection there are three plate sizes, not two. The long one is about 17mm, medium is ~14mm and the short is a folded ~11mm plate. I like them in their own way, and yes it all depends on your system, tastes, etc, but for me the medium length is the best. Here is a summary of some of my thoughts:
-short: very sparkly and present (upper mid-range hype) that appeals to a lot of people who first hear it. Nice for sure, but for me a bit too much like eating cake icing all day.
-medium: the most 'natural' of the group and with several variations (5814, 6189, various getter supports etc). My favourite. Can't go wrong with the variations. The 12AU7S can be breath-takingly clear but also the least 'warm' of the group. The regular 12AU7 (single getter support) is probably the 'warmest' with the most harmonics, but can lack some of the clarity of the others. The 12AU7WA 6189 with the double getter support is a nice balance between the two. But they are all great and this is one of my all-time favourite tubes, no matter what variant. Recommended.
-long plate: 'scooped' sound (more lows and highs, less mids). I have a soft spot for this style since it was one of the first decent tubes I ever bought. But, truth be told, it is probably the least audiophile of the three. But as I said, it all boils down to your system and your tastes.
RT was owned by Philips, but I have never heard a Dutch Philips sound like these (not even the famous 7316). The medium length is one of the most under-appreciated tubes around, with not a lot written about them that I have found, but they are getting very hard to find (at a decent price) which makes me think that people are quietly snapping them up.
Hope that helps...