With encoding/recoding/transcoding, the hard drive makes almost no difference. The speed is determined by the CPU, and is almost always WAY below hard drive speeds. Occasionally, with my 2 Raptor RAID-0 arrays, I can see transcoding speeds approaching 20MB/sec in Shrink with fairly low compression. This is well within the limits of most any drive. With higher compression, or with video format conversion, I usually see more like 7-10 MB/sec. In most cases, even using only one drive will not slow you down noticeably. If you want to speed it up, get a new dual core CPU with lots of L2 cache.
Back to your original question, sustained file transfers are usually limited by the writing speed of the receiving drive, as write speeds are lower than read speeds on the average hard drive. So moving a file TO the drive with the fastest write speed will be quicker in terms of MB/sec.