Why not splurge at the same time ?
Go rent the 70-200L IS II. At the same time, look at what camera you would consider upgrading to- whether it be a T4i, 7D, 5D3, and rent one of them as well. Please note that if you change formats- i.e. go to full frame- you will want a basic lens like a 24-105L as well- for the complete picture.
I mean, if you are gonna do it- go all the way.
EOS 5D III, EOS 7D,EOS Rebel T4i, Canon 70-200 f/2.8 IS II, Canon 24-105L, Canon 18-135 IS STM, 1.4x TC III, 2.0x TC III, Σ 50mm f/1.4, Σ 17-50 OS, Σ 70-200 OS, Σ 50-500 OS, Σ 1.4x TC, Σ 2.0x TC, 580EXII(3), Canon SX-40, Canon S100
Fond memories: Rebel T1i, Canon 18-55 IS, Canon 55-250 IS, 18-135 IS (Given to a good home)...