Which is a better choice on a development box if you primarily develop Asp.Net applications and SSRS reports. I have never had to use the Express editions, so I don't really know the pros or cons.
The cons I have listed for Standard+ editions are:
Pros:
Express isn't really designed for actual products - it is more to let developers familiarize themselves with the system and see if they like it, or for sample apps.
Express is free though, but if you need a free solution you might be better off with MySql.
Are you talking about for your dev machine, or for production? If it's your dev machine I would just pony up the ~$50USD for the developer sku, the only caveat is to make sure you don't make use of enterprise features unless you will have enterprise in prod.
I don't have experience with the 2008 versions as yet, but I've used both the 2005 and 2000 equivalent (MSDE) on live production projects. The codebase for both of these is essentially the same as the full blown product but with restrictions on ussage and the absence of some tools - the later of which can be generally worked around with 3rd party replacements.
If the number of concurrent users is low, and the the database is unlikely to grow that large, then generally the express versions are fine. It's a little more hassle to manage than having the full edition to hand, but the cost saving is significant.
Low and that large are of course elastic, but for example we have a real estate application that runs in several offices with half a dozen users and a couple of tables with a million rows and performance and management is perfectly fine.
SQL Server Developer Edition.
The only problem you have to watch out for is that it has features not available (it basically has the Enterprise Edition features) in the Standard Edition (for example, indexed views).
So, for instance, dev edition will work much better than express on your quad-proc dev box.
http://www.microsoft.com/sqlserver/2008/en/us/editions-compare.aspx
Usually there is no "Best" choice, however in this case there is: SQL Server Developer Edition. Reason: It gives you everything in every other edition (except the licensing) for all of $65-$90. Seriously, any SQL developer would be crazy not to get it, it's one of the best deals in the history of software.
Other stuff that you mentioned:
System Load: this depends much more of how you use it than what edition it is. I have both 2005 & 2008 instances running on my 3 year-old 2GB laptop with no ill effects.
pain to attach database for projects: painless with developer edition.
pain to detach unused databases: same
"You have everything you need": now you have everything that you will EVER need (for 2008).
Management Studio features: Yep
Easy move to production: Yep
Honestly, at this price, employers should be giving this to all of their developers.
On the rare occasions where I am in the same situation (usually on my personal web hosting), I opt for sql express + various free or cheap third party tools to substitute for SSMS and EM.
The development editions have all the features of the enterprise editions (with some quirky exceptions, consult BOL), so it's not like the dev edition will give you fewer things to play with. You just won't be able to let any of your customers connect to your SQL developer edition, you'll have to deploy your SSRS reports to a Standard Edition production server somewhere.
If you are using SQL at work and your employer doesn't have a license for SQL Standard at all, then that sound like a licensing problem.
In short, I think the choice of edition starts with a choice of license, which is an economic and legal question, not really a feature set question. (i.e. will your planned uses be legit with your chosen license? with SQL Express and Dev edition you can't wrong-- the sql express has you covered for production use, the dev edition has you covered for being able to use the features you want, like SSMS)