Might I recommend an alternative to both a rack bag and a saddlebag. The Vaude offroad 10+2 bag.
This little beauty has a permanent seatpin bracket (spares available for all your other bikes) and easily clips on or off. It has the capacity to carry all that you list (I do), and, offers no weight penalty over either of the other solutions, but, adds speed of mount / dismount, and great flexibility.
Wow that thing is almost identical to my Vaude rack bag! Unfortunately the rack bag has 4 velcro fixings that are a pain to constantly take off and put back on. I can see the quick release bracket of that would be very nice! I have yet to see how waterproof my vaude rackbag is as it hasn't really rained much in the 2 months I've had my new bike ... club ride this evening might test that out though
It is a great bag and it was a bargain secondhand from Sgt. Pluck of this parish. It comes with a separate rain cover though I haven't felt the need to use it yet. The side pockets are very useful for easy access to bits and pieces you may want to grab quickly like keys, purse, work security pass, energy bar(s), brevet card, etc. The bag is expandable in the normal way (a concertina section released by one zip) and has an elastic on top for carrying a showerproof / waterproof. It also has a light loop where I have installed a Smart Superflash to complement my dynamo lighting.
The bracket fixes to the seatpin with a metal band and is unobtrusive. The bag has a sturdy metal frame support which also forms the means of connectivity to the bracket. I have even leant on it forgetting that it was not a rack pack and it has withstood my substantial frame.
I carry my swim kit (including wash kit) in the bag. This was the original purpose for getting it.
My audax bike with this and an Ortlieb barbag has sufficient carrying capacity for what I need it for.