Both are good. Wind has a good data add-on called Mega1500 which offers about 500MB/month for 8€ (plus 4€ as activation fee), while 3 has the Naviga3 option which offers 50MB/day for 9€/month or 100MB/day for 3€/week (no activation cost). The monthly options are recurring (both Wind and 3), while the weekly option is not automatically renovated.
What is VERY important is to check if you have coverage in the area you are interested into and which kind of coverage you have. The offer by 3 only works on their WCDMA network, while it's not valid on national roaming on TIM (check
HERE for the coverage).
Wind has a wider coverage in GSM but smaller in WCDMA, so once again it would be good to see if you have WCDMA (which is of course much better for data).
Porting your number should not be a problem, but it can take some time. Since you loose your credit when you port the number to another operator, I suggest you to directly buy a new simcard.