Not only that, there doesn't seem to be any consensus between the pictures and descriptions of them in books (Baensch, Padilla, Rauh et al) either.

I'd like to know too.

Susan,
What little I know,I will try to pass on,having grown 2 of 3, testudo and arvensis.

Testudo: grows in an almost caulescent manner,with the stem lengthening and branching with rather short compact leaves,in a bushy shape,and has a black pointed spine at the leaf tip that is hard and sharp.(many 'ouch'!)The infl.is oval,and Neon Pink in color,really outstanding.Leaves medium green.

Arvensis: more the normal tank shape of bromeliads,a bit darker green leaves than testudo,but easily distinguishable by the underside of the leaf having a frosted look,with light banding.Leaves are longer than testudo,sharp small spines on the leaf edges,and the infl.is similar in shape to testudo,but often a darker pink,almost rose/red.

Quesneliana:(not grown by me)but from pics seen,seems to be a fuller plant,similar to arvensis,lighter green leaves,and with a taller inflorescence,dark pink to red,narrow and elongated.Probably the larger plant of the three.
Thats all I got.
bob

Thank you, Lisa and Bob.

I have at least two of the above, now I will have to decide which ones.

Also, I just acquired another Quesnelia with similar leaves as the others, but the inflorescence is branched.

bodiggly...if you want to post some photos of your Q's,we can try to ID them for you.
(or)
Q.testudo has that very sharp pointed, straight and black-tipped leaf. (a real dagger)
Q.arvensis has the lightly barred design on the leaf's underside.
bob

I have one called Q. quesneliana "Rubra" just to muddy the waters a bit. It has a reddish flush to the leaves. Same bloom as reg. Q. quesneliana though