Well the X30 will be phased out in few weeks. If you want to buy a good pda but don't mind to see it disapeearing from the market in few weeks and don't want the lastest software, go for it.
Also, Dell don't support anymore the X30 series. The last rom (A06) will be the last one for the X30. Also, no Windows Mobile 5 update for the X30.
So unless you have a very good price and you can't affroid the X50, I wouldn't even consider the X30.
btw I sold my X30 few weeks ago to get a X50v and I love it. Much better than my 4 months old X30. Beside price (and not much) there is no adventage to get X30.
Edit: As for battery life, I don't think there is any difference between X30 and X50..
X30 and X50v yes, but not the "QVGA" model.
For the X50v, a new 1800mah battery is coming soon and should give us way better battery life and won't "hurt" the design of the device