I too work for a large and well known consultancy but have heard of both Hedra and Serco.
I have friends who left to join Hedra and Serco and from what I hear they do not regret their decision.
They are involved in some large projects and
those at I don't think they have ever been pulled into a project they didnt want to be involved in.
Hedra have a great name in the Consulting space: particularly in the Govn space.
Serco are growing and have won some interesting projects lately.
Also my friends are always praising the culture and way they are recognsied by their firms...something I can not boast.
Working for a big firm can have its advantages (obvious) but as I get older I've started to recognise the value of having life. Also my friends are happy at these smaller firms while being well paid so maybe the grass is greeener...?
Great you have offers from both though! :-)