Afaik Spike only uses the FO guidance for long ranges.

EuroSpike has some detail improvements and an EuroSpike representative suggested to me that contrast is improved. There were some reports about Javelin being unable to lock on simple bunkers because the IR signature of the bunker was the same as that of the ground.

A disadvantage of (Euro)Spike is that the launcher itself cannot as easily be employed as IR observation tool as Javelin's sensor unit. That's iirc a purely mechanical centre of gravity problem.

It's difficult to compare both systems because the true and differences are hidden inside, not published and thus even unknown to those who know one of both systems well.