Front or back, I'd like to see the VOLT logo large over the SI logo.
If I were to put printed words on it, I'd want them small, and underneath the SI logo and the same length of line as the SI logo.
Nonetheless, I'd probably buy your t-shirt to at least wear around home. Super big print isn't something I'd wear a lot of places.
Or, heck, I'd like my Volt/SI banner on a shirt, but it would have to be custom, I suppose, unless you were to put something like "I quit smoking tobacco products. Ask me how much better I feel and how long I've been vaping, instead." or a shorter version, "I quit smoking tobacco products. Ask me about vaping, instead."