There are many factors that affect the loudness of a patch, many, MANY MORE than just the volume fader.
First of all, who knows the volume of the original synth guitar? Who knows which mic was used to record its amp, how loud that amp was, which speakers were used, at what distance the mic was, how much the room reverberated, how the gain was dialed in on the preamp, whether or not the engineers making those samples processed them or later adjusted the gain or normalized the samples, what other processing effects there are in that channel strip's inserts and how they affect the gain? Now ask yourself the same questions for the other patch.
And even if ALL of those were exactly equal.... in real life, what sounds louder: a fuzzy synth guitar or a choir sound? And do those instruments have a dynamic? Meaning: can they play softer and/or louder notes? Do they have the same dynamic? Why? Why not?
That's a lot of questions, and the goal wasn't to research the answers to those questions, just to expose the fact that you really can't expect two instruments to have the same perceived loudness just because their two level faders are in the same position.
Now back to what really matters: simply adjust your volume faders with your EARS, not with your eyes, in order to match the perceived loudness of the two patches as desired.
Matching patch levels is not as easy as it may sound. I've done it for live shows, and the relative perceived loudness would change from one venue to the next. For example in one venue sound A was way too loud and sound B was too quiet, but in another venue it would be the opposite!! That's because the acoustic of the venue where you play also affects the way it amplifies certain frequencies or others, etc...