Ever since the 61-series, Lenovo made a peculiar decision to eliminate almost all difference between the 14" R and 14" T models. The 14" R series now use the same case materials, the thickness difference has been reduced to almost nil (basically to accumulate the Ultrabay Enhanced), and the weight difference is marginal at best.
Interestingly, the 15" R models have continued to be bulkier and heavier, using lesser materials (though not necessarily less durable). There are also some differences in feature sets. For instance, 14" R61 with Intel graphics can have DVI out, 15" R61 with same configuration - cannot. 14" R uses Intel Ethernet, 15" uses the cheaper Broadcom chip, and there are a few other differences escaping me at the moment.
Regarding materials, here is your answer, straight from the Tabook/Ltwbook:
T61/R61 14W/T400/R400/T500/W500: Top: Super-Elastic PolyCarbonate (SEPC); Bottom: Carbon-Fiber Reinforced Plastic
R61 15W/R500: PolyCarbonate/Acrylonitrile-Butadiene-Styrene (PC/ABS) plastic
Previous (R60 and older) models used ABS for both 14" and 15" models.
Weight (with 6-cell battery):
T400/R400: starting at 2.3kg
T500/W500: starting at 2.7kg
Clearly, the 15" R and 14" R are very different machines now, with 14" R being virtually indistinguishable from the T, when equivalent configurations are compared.
Current: T410 2537-R46, T60 1952-F76, T60 2007-QPG, T42 2373-F7G, X61 7673-V2V
Collectibles: X32 w/ IPS Screen, A31p w/ Ultrabay Numpad
Past: Z61t 9440-A23, T60 2623-D3U, X32 2884-M5U