Getting a really good acoustic guitar sound out of a keyboard can be a real challenge. Generally speaking, if I'm playing a chord using an acoustic guitar voice and I want it to sound like a guitar, I try to play the chord with a rolling motion of my hand-- that is, play the leftmost note of the chord first, followed quickly by the next note of the chord, then the next, then the next, until you're holding down all notes of the chords, so it sounds more like the way it would if a guitarist were quickly running the guitar pic across the strings to play the chord. It might take a little time to get used to playing chords this way, but it's not too difficult. I think there are YouTube videos of people showing how to use this method to get a more convincing acoustic guitar sound on a keyboard. Otherwise, if you play the chords the way you would normally play them on a keyboard-- trying to strike all keys of the chord as close to simultaneously as possible-- the acoustic guitar voices will tend to sound sort of like a piano voice.
As far as arpeggios, I'd like to make two comments.
First, the arpeggios which are programmed into a keyboard will be very precisely timed, which can make them sound "robotic" or as if a robot were playing them. For some types of music, such as electronic music, that might be exactly what you're trying to achieve. But for other types of music, it usually sounds more convincing if you can play the arpeggiated chords and arpeggiated phrases manually, because then there will be all sorts of little "timing variations"-- I hesitate to call them "timing errors"-- where the notes being played don't always occur infallibly on the precise down beats, up beats, quarter beats, or other fractions of a beat. These little imperfections or variations in the timing will make it sound like a human being is playing the music, rather than a robot or computer.
And second, I think the answer to your specific problem can be found in the Function menu. If you look on page 46 in the Owner's Manual, at the very bottom it lists the "Pedal Function" setting, which controls whether the foot pedal will sustain the notes you've pressed when you lift your hand, or will hold the arpeggio you're playing when you lift your hand, or do both at the same time. See page 20 for a slightly more detailed explanation of the "Arp Hold" setting. Note that you'll need to have a foot pedal or foot switch to make use of this feature. If you have the Survival Kit which is designed for the PSR-E363 (which I think would be the SK B2 version of the Survival Kit), it includes a foot switch that you can use as a sustain pedal. However, the FC5 foot switch or FC4A foot pedal should be more sturdily built and durable than the foot switch that comes in the Survival Kit.
Now, as far as your question about how useful the preset arpeggios really are, this can depend on how you're trying to use them. As I said, they do tend to sound a bit "robotic," so they can be best suited for various types of electronic music. However, the Arpeggio and Harmony features can be very useful for playing certain types of voices, such as Steel Drums or Timpani, or Acoustic Guitar, or Harp-- any type of instrument where a sound is rapidly repeated (such as Steel Drums or Timpani), or where a series of notes are played in repetitive sequence (such as Acoustic Guitar or Harp). I have a lot of fun experimenting using the various arpeggios with a given voice, especially by finding an arpeggio that I like and then adjusting the tempo to speed up or slow down the rate of the arpeggio.
Note that the Arpeggio and Harmony features are closely related to each other, which is why you can use one or the other, but not both at the same time. You could say that Harmony like a simplified Arpeggio, although that isn't quite right, since they behave a bit differently.