"No life" is a function of the PMs you are working for and the culture that pervades the practice - I'd say that you're as much at risk of being hit by this at a small firm as at a large one. In a small firm you'll have periods like this because in small firms everyone "pulls together" when needed; at a large firm it's just a function of the project you're on and how much this is considered "the norm" by those you are working with.
Given the above, I'd therefore say the decision comes down to earnings potential, risk and challenge / exposure. My take - not knowing the specific firms you are weighing up - would be:
- no surprise to me that earnings are higher at the small firm. I would also usually expect smaller firms to be able to fast-track the promotion of strong performers at a rate that larger firms cannot match. The latter are usually bound by time in service "promotion norms" that make it harder for them to make exceptions for exceptional performers. So assuming you are and continue to be a high performer, potential earnings in the next years are almost certainly higher with the smaller firm.
- the flip side of this is risk. Large firms do sometimes have to let people go if there's a really severe downturn or a particular practice (think public sector recently) sees its pipeline of work decimated by a change in the market environment. But by and large a strong performer is unlikely to find themselves made redundant if joining a large firm. By contrast, small firms are usually overly reliant on just a handful of key clients. If just a couple of these slam on the consulting-spend brakes for any reason, that can cause major problems at the firm. I can think of one firm I know that went a full quarter without billing a single billable day during the recent downturn - at which point job losses for strong performers become inevitable. So you need to do more due diligence on the strength of the client portfolio the smaller the firm you are considering - and accept that the risk here is invariably higher. Depending on your personal circumstances, this may or may not be a risk you're happy and able to take.
- challenge / exposure is not something that is so easily defined in broad terms. A lot of this will come down to the amount of practice leader / partner / client exec contact you'll have in the roles and the calibre of the key individuals you'll be learning from. There are also different aspects that firms may be able to offer. At a small firm your involvement in lead generation, marketing, proposal writing, etc. is likely to come at a younger age; but the scope to work on international engagements or projects outside your specialist sphere is probably more limited. The calibre of clients across the two firms may be similar but that doesn't mean the engagements being worked on are equally prestigious / challenging. This too needs full consideration.
Not a definitive answer either way I appreciate, but hope this helps you in considering your options - and well done on securing the two offers.