Linux, total disk space

I get the following:

-bash-3.2# usermod -g mqm -G mqbrkrs mqm
-bash-3.2# su - mqm
-bash-3.2$ exit
logout
-bash-3.2# df -hT
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/sda1     ext3    9.7G  3.8G  5.5G  41% /
tmpfs        tmpfs   1006M     0 1006M   0% /dev/shm
/dev/sdb1     ext3    9.9G  3.0G  6.5G  32% /opt
-bash-3.2#
-bash-3.2# du -sh /
6.7G    /
-bash-3.2#
-bash-3.2#
-bash-3.2# fdisk -l

Disk /dev/sda: 12.8 GB, 12884901888 bytes
255 heads, 63 sectors/track, 1566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1        1305    10482381   83  Linux
/dev/sda2            1306        1566     2096482+  82  Linux swap / Solaris

Disk /dev/sdb: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1        1305    10482381   83  Linux
-bash-3.2#

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So the df -hT says I have about 10GB per disk, and the fdisk -l says I have about 12 GB in the sda and 10GB in the sdb

Why is there a difference?

Which of these 2 commands whould I rely upon

Well, I would say fdisk is.  It gives you the sum total of the entire disk whether it is usable disk space or not (un-partitioned).