Hi Symfoniq. Currently, Symphonic Cube (Full version) and SE Complete Bundle seem to be more or less the same price. The SE Bundle instruments are in the main cut-down versions of the full collections found in the Cube, with less velocity layers and tone-interval (rather than chromatic) sampling. However if you audition them side by side, the difference is fairly negligible for the most part.
SE Complete Bundle contains certain instruments not found in the Cube:
STRINGS
Solo Strings & Chamber Strings con sordino (with mutes) samples, Appassionata Strings
WOODWINDS
Bass flute, Oboe d’Amore, Heckelphone, Basset horn, Contrabass clarinet, Bassoon 2
SAXES
Soprano, Alto, Tenor, Baritone & Bass saxophones
BRASS
Trumpet in Bb, Fanfare Trumpets (6 players), Cornet, Flugelhorn, Alto trombone, Euphonium
EXTRAS
Vienna Choir, Bösendorfer Imperial Grand Piano, Vienna Konzerthaus Organ, Harpsichord, Electric guitar (Overdrive), Concert guitar
The only one in the list I would badly miss is Appassionata Strings, which has the lushest sound of all the VSL string sections.
Personally I don't think MIR is essential; it's just the icing on the cake - the *samples* are the cake! MIR is obviously very effective, but I would check out other makes of reverb before you commit to it.
Dimension Strings is a great tool for specialist string arranging and pointillistic, individual instrument performance-tweaking, but it would be a challenge for a newbie - arguably best to get familiar with the huge number of articulations in the regular VSL string libraries (which can produce fabulous results) first, before diving into the deep end with DS.
Others will disagree, but I'd say buy the Symphonic Cube, get familiar with the instruments and articulations, then maybe add other specialist instruments on a piecemeal basis as and when you need them.