Liberty or adventure? May '12 western med

[gooner21]

hello all,

very keen to experience a cruise for the 1st time in life. my wife in me both mid 30's along with our son who would be 4 and a half yrs old will be traveling and we maybe maybe joined by friends. we've assumed a 7 day cruise would suit us best and have decided to go for the western med region.
the options we've found are as follows:
1. $649 - adventure of the seas from malaga (2nd may '12)
2. $1049 - mariner on the seas from civitavecchia ( 9th june '12)
3. $1249 - liberty of the seas from barcelona ( 2nd june '12)

now i hear the liberty is the best amongst the 3 available options but almost double the cost of adventure.
also we prefer going in may than in june (no liberty option in may) but if some experienced travellers would be kind enough to inform me if it is worth paying such a huge difference in amount for the liberty then we would go in for it and also change our plans to june rather than may. also since we've travelled before quite a bit of europe a number of times, our focus is not necessarily on the ports we are docking at but moreso on the quality of the cruise experience. our focus is to make this a fantastic experience for our son as well.

also my other query is there a big difference in the sleeping/stay experience between the inside cabins to the outside or balcony?

your valued inputs would go a long way in making our 1st experience pleasurable and memorable.

thank you!