The X5 will give you 3200dpi resolution for medium format. Estimates of the V750M's actual resolution vary between 2400-3000dpi. In any case, noticeably less detail is visible at 100% on screen (not to be confused with the need for sharpening the V750M scans more than the X5 scans). Whether this actually becomes apparent in print depends on your target print size, and in my experience the resulting differences in resolution are rather academic at A2+ size (Epson 3800).
The key differentiator between the X5 and V750M however is dynamic range, and here the X5 runs circles around the V750M. The X5 can still detect detail in highlights and shadows (and fine tonal variations in general) where the V750M only sees black and white patches. This is less of an issue for color negatives, but if you have color slides or black&white negatives, I'd clearly lean towards the X5 rental.