Depends what motivates you, note that I haven't worked in ACN, I have had friends who've served time and worked with the firm. I've never heard of the other firm, but then I'm in the UK.
The generic points relate to the small niche, big SI balance. In ACN you're a very small wheel in a very big machine. In theory the larger firm should allow a reasonably structured progression, development opportunities etc. ACN doesn't get a good reputation for that. A niche firm is less able to do that so you'll get more formative training on the job, the quality of the training depends on the coaching and mentoring skills of the more senior consultants that you'll be working with.
In terms of work, at the big firm then you'll be pretty constrained, there isn't much opportunity to do your own thing. I know that personally I don't really like the hierarchical culture, I don't respond well to be closely constrained or supervised and as a result in the past I've clashed really badly.
The smaller firm is more likely to give you more responsibility. The thing to be aware of there is whether the culture allows you to make the corresponding mistakes, which will happen.
Personally I'd go for the niche option, but my risk appetite suits that and I'm not in need of the structured development any more.