Without knowing what you plan to shoot, it's pretty difficult to give a definitive answer of which option would suit you better as both cameras have advantages and disadvantages over each other.
Perhaps you could tell us what in particular you think is wanting with the T1i?
Personally, I'm on the brink of upgrading my 550D (T2i) to the 70D because I'd like an articulated screen, a touchscreen, the 7fps drive mode, the superior autofocus for fast-moving subjects and the compatibility with EF-S/EF-S-compatible lenses.
Whilst they'd be nice to have, the benefits of better noise management and AF sensitivity in low light and a brighter viewfinder aren't enough to tempt me to go to full frame yet, particularly as I'd have to re-invest in two of my lenses.
Your circumstances may differ from mine so your reasons for wishing to upgrade may also differ.
DSLRs: Canon EOS 7D Mark II, Canon EOS 70D
CSC: Canon EOS M3
Lenses: Canon EF-S 15-85mm f/3.5-5.6 IS USM, EF 50mm f/1.8 II, EF 100-400mm f/4.5-5.6L IS USM, Canon EF-M 18-55mm f/3.5-5.6 IS STM, Canon EF-M 22mm f/2.0 STM