Performance - Ranking
Since the ranker groups products only by processor class, we need to scan the SoC-1 (single core application-specific processors) class rankings to find the relative ranking for products with the same number of bays. For four-bay RAID 5 products, we find the TS-420 in the #4 slot, which is the best for four-bay NASes in this class. Surprisingly, the TS-421 ranks at #11 (tied with the TS-121), with Synology's DS413j as the only other four-bay NAS between them.
Put another way, the TS-420 is in the #1 slot for four-bay SoC-1 class NASes and the TS-421 is ranked #3. So why? Comparing the Ranker Performance summaries, the standout differences between the 421 and 420 are Windows FileCopy write (45 vs. 53 MB/s), Windows FileCopy Read (65 vs. 79 MB/s), NASPT FileCopy Read (59 vs. 75 MB/s), Video Playback (71 vs. 95 MB/s) and USB 3 - NTFS backup (46 vs. 28 MB/s). In the 421's favor, it slightly outperforms the 420 on many of the other benchmarks. But when the performance goes the other way, it goes big, pushing the 421 down in the ranking.
NAS Ranker Performance summary QNAP TS-421 & TS-420
Among all SoC-1 class products, the TS-220 at #6 outranks the TS-221 at #7. Looking at only two-bay NASes, the TS-220 and TS-221 rank at #5 and #6. The differences this time are less striking and amount to small differences in numerous benchmarks that all add up. Again, however, it's the less expensive and supposedly lower-powered TS-220 that comes out on top.
NAS Ranker Performance summary QNAP TS-221 & TS-220
Finally, for single bay SoC-1 class NASes, the TS-121 at #11 (tied with the TS-421) finally outranks its less-expensive TS-120 cousin tied at #14 with the older TS-412. Among only single-bay SoC-1 class products, the TS-121 ranks first, with the TS-120 third-ranked with the older TS-112 between them.
NAS Ranker Performance summary QNAP TS-121 & TS-120
Looking at the Ranker Performance summary comparison above, the difference in ranking again comes down to small differences that all add up. The biggest difference that jumps out at me is the TS-120's 86 MB/s Video playback vs. the TS-121's 77 MB/s. Otherwise, these two NASes are about the same as you can get.
QNAP is known for having a lot of NASes to choose from. And in this case, it just seems like too many. By our testing, paying a premium for the TS-x21 series with its slightly higher processor speed and 1 GB vs. 512 MB of RAM just doesn't seem worth it. The premium you pay depends on how well you shop. But it can be significant in percentage as Table 3 shows.
Table 3: The x21 premium
(all prices from Pricegrabber on 7/29/13)
There may be scenarios where paying the premium might be worth it. But if it were our money, we'd opt for the x20 versions and pocket the difference.