Windows 8 is a touchscreen optimized OS which does not work as well on desktops with mouse and keyboard. For users who want the traditional desktop experience it is often frustrating, which is totally unnecessary since Win7 provides the perfect desktop experience and shows no signs of age.
I would base my upgrade decision on which of the actual features you may (or may not) need in the higher version.
Compare Windows 7 Editions
Otherwise they are the exact same OS.
Since Anytime Upgrade is not available in Windows 7 now you'd need to buy a Professional key to insert into
Windows Anytime Upgrade - How to to unlock the few extra expensive features you know for sure you need. The best prices I see for these are on Tiger Direct which regularly has Pro OEM for $135, Home Premium for $89. New Egg also stocks these.