My first post on this new board, exciting
Anyway, here's my take on this topic, as i've tried and looked through both board types source codes, and made small features for both BB packages.
The Phpbb group had their thinking caps on the whole way. Their PHP and server-scripts (the cogs of any BB software) is far superior. Not to mention they designed a pretty extensive easy to use template parser, which pretty hit the template engine goal bang on (not too over the top with a completely new syntax, but with plenty of easy to use features).
(As a side note, I'll be releasing my own PHP template engine very soon, which uses permission tags in an ever more familiar sense)
I could carry on praising Phpbb3, but I'd rather get on to why MyBB is inferior.
First off, MyBB stores its templates in the database, which on its own isn't necessarily a bad thing, however the deal is, they are querying the database everytime for a new template, which on a BB board that requires queries for A LOT of things, you'd kinda want to cut down. If your going to be fetching quite a few templates fetch all the ones you need in a single statement and just sotre them in an array.
SELECT title,value FROM template_table WHERE title = 'index' OR 'whos_online' OR 'forum_list'
Getting all three [example] templates at once, loop through results storing them in a $templates array.
So my first point, there template system is expensive on loading speeds.
Secondly, they aren't using replacements for their template "Variables", but instead CONSTANTLY, and I mean A LOT, using Variable Interpolation, which is requiring all strings to be parsed for variables. Simple variable concatenation is much less expensive.
And thirdly, these template string are having to be evaluated once pulled from the database, in order for these variables to be inserted in the string. And well, the eval() function is well known for being a bad thing.
Anyway, personally I'd save my database the grief of MyBB, and would use Phpbb3.
Feel free to disagree