The legato from the full library does not sound better, as it contains the same samples as the SE, but:
1: In SE you only got legato and portamento (leg.sus uses the same samples as legato), and there are exactly two samples from each note to another (one mezzopiano and one mezzoforte). In some cases the legato transition is VERY characteristic (for ex. an audible key click noises in wind patches, portamento-like transitions in the strings). If the same combination of two notes occurs more than one time in a piece, then you will always hear this prominent sound.
The Full library offers a fast legato, zigane-style portamento, legato sul (on one string), and of course the perf_trill, so you can always choose from several similar sounding alternatives to create little differences
2. You probably know, that the Full library is chromatically sampled, you get samples for every note, while the SE only gives you every second note, the semitones inbetween are produced by playing the samples faster. This is not audible for a single note, but when you alternate between two notes which use the same sample, in some cases this can be noticable.
3. Some articulations offer more velocity layers in the full libs.