How does LMM/MM banking compare to BB/EB banking from a workflow perspective?

Full disclosure: I've never worked at a BB, just boutiques and a MM shop. That said, I work with plenty of people who came from BBs, so here is my second-hand take on the biggest differences.

The bulge bracket IB model is sometimes referred to as "whale hunting". The idea is that bigger, more prestigious firms should chase bigger, more prestigious clients instead of "stooping" to smaller middle-market deals, so BBs do more pitching. The key reasons are that (1) there's inherently less work to go around when you're focused entirely on large blue-chip clients, because the universe of companies that big is limited; and (2) when the universe of available clients is limited and you're still competing against other massive, well-renowned BBs, senior bankers have to do everything they can to stay in front of their potential clients, so there's a clear incentive to put together BD deck after BD deck with new ideas.

The middle market IB model is far more predicated on volume, because the middle market is massive (i.e. way more small-medium sized companies than huge companies). The MM private equity universe is also much more saturated than megacap PE, and they all have mandates to put capital to work, so both the supply and demand of M&A opportunities is extraordinary - and, generally speaking, more insulated from the ebbs and flows of large cap M&A activity. 

So TL;DR, MM IB is more execution-focused than BB IB tends to be. For that reason, you could probably conclude that BBs are more modeling intensive (because valuation modeling is more prevalent at the pitch stage and merger modeling would be most common for large cap deals anyway). Not that MM IB lacks modeling exposure - clients tend to be less sophisticated and therefore bankers are both more hands-on and more creative in how forecasts are developed. As for what's more or less "rigorous", I guess it depends on what kind of work you think requires the most rigor; it's not going to be smooth sailing at a MM shop relative to a BB if that's what you're wondering.

Cultural intensity varies significantly from firm to firm (whether BB, MM, or boutique) and even group to group, and in some cases might have more to do with where geographically you're based than which firm you work for. I've seen marketing materials (CIPs, etc.) from a number of MMs and BBs, but haven't noticed a clear pattern for whose work products are better - that probably depends the most on how your MD likes to run deals, even though most banks have templates for how they like certain materials to look. I will say that junior bankers, in my experience, have more banker and client interaction at MMs than BBs because the deal teams are smaller and there isn't as much cross-collaboration between industry/product groups.