If you look at the data sheets (100D vs. 200D) you will notice that even firewalling throughput on a 100D is rated down from 2.5G for large packets to .2 G for small packets (like HTTPS). This is caused by the lack of a dedicated network processor ASIC (NP) on the 100D.
Second difference is the CPU used, an ATOM vs. a Celeron. Session buildup and (some of the) SSL decryption is done via CPU, and here the 100D shows poor figures: "new sessions per second: 22k vs 77k".
And even for UTM, e.g. the AV throughput is x2 higher on the 200D which is 600 Mbps. Deduct a lot from this figure if you plan to use AV, IPS, AppCtrl at the same time.
If you can afford it the 200D will suit your current needs today, and may be the better investment over time. Both of which I would disagree on with a 100D.
PS as for the comparison to Sophos: from the datasheet, it seems to me it should be compared to a 200D, same CPU, comparable RAM, interfaces etc. Letting it run against a 100D is a bit unfair. Though this is an economical question as well...
"Kernel panic: Aiee, killing interrupt handler!"