Completing ATA will get exemption for ACA.
For job security, it's safer to go with the big 4.
If you are from a average uni with only a 2:1, I will stick with the big 4 because you are lucky to get in. On the other hand, from top uni & 1st class, take the risk and go for the small firm. Once you qualified as CA, you can apply to the big one again.
Please remember, small firm will deal with small client, but jobs do vary. Big firm, monotonous work before you qualify but you will have the opportunity to move to different department.
I hope the above will help.
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