I believe assessed minis have greater weight on forms by virtue of being more exclusive--a pupillage-limbo, so to speak. Simon seems to disagree, but then perhaps this is a reflection on his set's commonsensical liberality more than the status quo at London sets. I'm currently applying for all assessed minis I can find that are not entirely incongruent with likely OLPAS choices--I'm finding it difficult to find them, though, other than at the MC/Silver sets (any suggestions?).
I think it's imperative for anyone who is (as they well should be) applying for both London and provinces sets to have at least 1 mini within each circuit (I've done two in NE, will be doing one in the South shortly), to show that their pupillage applications are not motivated by desperation but rather by a consistent pattern of interest. A vac placement in the provinces is absolute gold, too (even if it involved me crying bloody tears of boredom for a week), as is a London placement (ahh, Olswang's fine assortment of jelly beans). It's also key to get minis at specialist sets if you'll be applying there (i.e. my ideal set is a media one--unfortunately, as there are only 4 of them, this means I need to do a mini at each lest the remaining sets become upset with my choices). Also, without having seen the OLPAS form, I'm thinking of putting the number of mini pupils recruited annually by the set (where advantageous), so as to get across the fact that they aren't just handed out like candy.
I entirely symphatise with your 'what if i get an offer' quandary--I'm starting BVC next September, having already graduated....my advice there would be to look at just how many offers/interviews you receive--if it's just one, then it's likely a lucky fluke--grab onto it as if you life depended on it. If not, then you can obviously repeat the performance the year after... My concern there, however, is that using the 'hail of grapeshot' method in applications (i.e. applying to all sets you could possibly live with practising in) prior to the BVC may 'black list' you, to an extent, during the subsequent season. Simon's recent entry suggests this is the case--however, a few other barristers suggest it isn't. Personally, I don't see the rationale in penalising persistence and self-improvement...
Regarding mini--> pupillage, it absolutely helps (even if just to show you were interested in that set not out of desperation alone...). I tend to get along very well indeed with my shadow-masters--record your impressions, court visits, discussions for future use at interview. Also, if at all possible, let the clerk know you admire the work of *X Senior Barrister at that Set who is well known for Y* and would love the chance to see him in action, naturally if this is not inconvenient. You'll not only get the points for proactivity, but also get exposure to more interesting work and can chat up someone who is far more likely to be on the pupillage committee than a junior barrister--also, their opinion would carry more weight.