Normaly aluminum has less flex and is stiffer in general. But with that being said it all depends on the material and how the tandem is built, this applys to both materials aluminum or steel. A good quality steel tandem can be stiff and have a great ride with no flex like the Co-Motion Primera you guys road. I have heard good things about the Burley aluminum frame from others it may do the trick for you. To bad you can not save up the dollars and get the Co-Motion Primera you liked, you guys would probably be money ahead in the long run if you could come up with the exstra cash. Also we chucked our suspention seat post years ago.
The Trek T1000 is a nice Tandem for around $2400.00 retail and has a nice stiff aluminum frame with good solid componets, you can probably find one for less than the $2400.00 retail if you shop around. I have seen them as low as $1700.00 to $2000.00
Good luck and ride together,
Bill G