Depends on what type it is.
Old Turbo memory is becoming useless (although it's ok for returning from sleep/nibernation) since it's so slow 40MB/s (much slower than an HDD buffer), but the newer turbo memory is much faster getting closer to Sata1 interface speed ~ 170MB/s, but as we move towards faster SSDs Turbo memory won't matter.
Robson 2 is ok, but between it and more system memory I'd still take more system memory, since the benefits of Turbo memory are very limited. But if you already maxed your system memory and it's a small additional price then Robson2 is ok, but if it's Robson 1 then it might be near worthless (most people like myself disable it now that there are fast 5400RPM drives Turbo memory actually holds you back, and can be unstable).