For the money, anything that normal Satellite TV use would suffer from?
I'm not privy to the official STB6110 specs, and while I have never put a TT-1600 on the test bench, I've communicated with someone who did. If all you do is DVB-S, the STB6110 will work fine. DVB-S2 8PSK may suffer a bit of sensitivity loss. With 16PSK and 32PSK the noise will become a serious limitation. Now there's no guarantee the STV0903 will lock either 16PSK or 32PSK, and there is not a lot of use of those modulations in North America currently, but people in Europe have had some success with it.
Separately I would also offer caution regarding any off-beat tuner on eBay, especially one with a limited or non-existent track record. I did not research this model and I know nothing specifically about it.