I have ESXi runing on a RAID 5 made of 3x250GB
I'm going to replace the 250GB HDs with 1TB disks
I can create in the RAID adapter another disk using the extra space or I can extend the existing one (to have Disk 1 = 3x250 - 250 and Disk 2 = 3x750-750 or one Disk 1TBx3 - 1TB)
What is better from ESXi point of view ?
You can not extend an existing vmfs datastore. If you have the additional slots, you can add the disks in and create a new datastore. Or, you can remove and insert one new disk at a time, wait for the RAID to rebuild, and then change out the next. Once they've all been updated. You can use the new space to create a new datastore, but still, you can not extend an existing vmfs datastore, per se.
With ESXi you won't have an option to expand an VMFS datastore but you can add an extent to make a single datastore larger, but in general this should be avoided. One thing to keep in mind for the future is that you'l want to keep the array that you present to ESXi smaller that 2 TB. If you have space to copy the VMs to other storage, I would backup the config of ESX and backup the VMs, swap in the new array and do a reinstall of ESXi. Then restore the VM files and the ESXi config.