I would go for the HALO. As someone else has said, if you upgrade your gun you can use it on any other marker (almost). Your gun is pretty basic and inefficient, so to me it's likely you will upgrade. People (I include myself in this) spend too much time trying to make crap markers good. I love the underdog, the the truth is, spend the money on getting a better base marker... and take the HALO with you
"Differences between empire and invert hopper" ... please specify the models you are referring to
BTW... if you decide on the ripclip, make sure you get the Empire TM series version. It is much more sound sensitive than the earlier versions.
As for the V35.... there is no reason that he should get a V35 for the BT slice, it will never ever be able to fire as fast as the hopper can load. Only worth getting if as mentioned he buys another better gun later. (not to mention most place will never allow such fast fire rates). (For reference I have a V35 hopper so I'm not dissing it, it's very good).
BTW... I don't know why you are comparing the ripclip and the halo, the halo is much cheaper 2nd hand than the rip clip. I'd be looking at something like a 2nd hand HALO Too, Scion, or the original Prophecy for the same sort of price. (all still cheaper).