Spend yer money on adding another Mic to your collection.
I'm happy with the Tascam preamps on my fw-1884. That said, I have a focusrite preamp/channel strip (2 ch.) because my best attribute is my voice. So, I want to treat it to the best I can get my hands on. The TwinTrak Pro is feature laden such as variable input impedance (allows me to tailor the response of my mics - very cool), 'Air' switch, Mid Scoop EQ, Optical Compressor, Phase invert (I need it for the way I decode M+S Recording sometimes), SPDIF In, F/X Sends & Returns, plus a nice monitor section.
How important are the vocals in yer music to this point?