Wombatspeed

Hi everyone. While this is my first post to the forum I have checked the archives for similar questions. While there is lots of info (thanks everyone) on frame flex and different frame materials and builds I couldn't quite find the answers. Here goes:

Main Question: Has anyone directly compared the steel Burley Rumba/Duet vs the aluminum Tamburello/Tosa? Are there any substantial differences in flex along the top tube (fishtailing)?

Background: My wife and I are new to tandeming and are currently test riding in the $1500 range. Combined weight around 260 pounds. Each doing around 50 miles per week on our singles. Captain 5'10'', stoker 5'2''. Both spin pretty smoothly, as spending a fair bit of time on rollers over the winter.
Prime motivation for looking at tandems: pulling our toddler in a Chariot trailer, and later, if she is up for it, having her stoke with kiddie cranks.
Secondary Motivation: New toy for fast(ish) 1-4 hour rides for the adults. Trying something different. Why wait until our daughter can stoke if we get some fun out of it in the meantime.

Experience so far: Ridden a Rumba and a Paso Doble. Size small fits great. Tried the Paso to see what additional $1000+ buys you. Sure it's a bit nicer, but the bottomline for us was that both frames seemed to flex like crazy along the top tube, almost as if there is a pivoting point between captain and stoker. While never really disconcerting, the wobbliness was quite annoying particularly when doing smooth S-turns staying within a lane of traffic, but even when cruising along the flats. Given our low weight we were very surprised about the wobble as we didn't expect to be able to flex any decent frame. (Though subsequently I heard from a similarly built neighbor that they called their recently sold Duet "Team Tuna" as it seemed to propel itself like a fish.) But with the amount of perceived flex we knew we would not enjoy riding this tandem.

To see whether the flex was because of us being newbies with complete lack of co-ordination we then tried a Comotion Primera (again size small). There was absolute no flex even when we tried to weave around hard. My wife said that this rides like a bike, not like a drunk tanker. It was just solid, and for the first time we thought that we could really enjoy tandeming, not just for its novelty value and for family duties (pulling trailer, having kid stoker). Note: We rode the Paso again right afterwards, and wobbled again big time.

The Problem? We don't have $3000 for a Primera. Would a cheaper aluminum bike fit the bill? We could order a new 2005 Tamburello for 1400, but can't test ride it. We know the small Burley geometry fits very nicely. However, this would only be worth it if the aluminum frame (with the same geometry as the steel Rumba) is likely to eliminate most of the top tube flex. Will the different material alone (since it's the same geometry otherwise) be able to do that?
Any thoughts and suggestions much appreciated. What other bikes in the 1500 range may fit the bill?

Final note: Comfort seems to be less of an issue. Even without a suspension seatpost on the steel frames my wife didn't seem to mind anything short of real potholes. So I would think an al frame with a shock post could work for us comfort wise.

Many thanks.