A few points for your consideration comparing these two DACs...
The Rowland Aeris decodes PCM only (no DSD), and can be upgraded significantly with their optional PSU
The T+A DAC 8 DSD decodes PCM and DSD, but reportedly excels with high rate DSD
I haven't ever directly compared them but am familiar with both. Don't assume a newer DAC design will outperform an older one, like most things in high end audio there are generally trade-offs between different designs. If you have all PCM files and don't plan on getting into DSD then the Aeris may be the better choice since it is optimized for PCM. The DAC 8 DSD provides unusually good sound quality with high rate DSD files, but more average quality with PCM decoding. Most T+A users seem to prefer their PCM upconverted to high rate DSD using HQ Player so that the DAC 8 DSD handles the conversion to analog from DSD which it excels at. Running HQ Player to perform such high rate DSD conversions with their most advanced filters is very resource intensive and requires a powerful PC, even in some cases also using additional processing power from a high end graphics card.
So to get the best out of the T+A would require a commitment with additional hardware for your server compared to what you probably already have. From what I've heard the new optional PSU from Rowland provides a significant upgrade to the performance of the Aeris DAC, and while not inexpensive could be a way to improve the DAC setup you already have in place.
Hope this helps at least a bit, and please let us know how you decide to proceed. Best of luck and good listening!