If it is not too complicated, can you explain why letting the wallet choose is better?
because it complicates things, you selecting outputs manually, calculating if it is enough, making sure the transaction is confirmed that you are spending, and generally things like that.
Also held down CMD (on a Mac) and selected the coins to spend, and both showed up in a "From" box on the send tab as you said. I stopped there. Address A was 2 coins and (change) address B was say 0.84 and I wanted to send you 2 coins even. If I did go through with the send, would it automatically send you 2 coins from address A and take the fee from the change address B? If Electrum starts by taking out the fees first, I would end up with change in A and B would be empty. A little arcane... and a tad OCD perhaps!
this is how a transaction and fee works:
when you spend bitcoin you are spending transactions, these transactions have a value. for example here is my bitcoin address:
https://blockchain.info/address/1PHLkjiXvvUfw7AtZoPC6hhnNg6jZ836kw0a927f86e75129a312d1e756cc1ded476fae6855aadec801c9885ed7c2f290f0 has a 0.03 value
13a4c5470acd599743073873294d64c5234a98281b4df9db9c0ec9a5471ad6a0 has a 0.0225 value
and so on.
my total balance is currently 0.16023 BTC
if i wanted to send you 0.01
BTC i usually choose one of these outputs (for priority the oldest one is chosen = e506d3..... which is 0.035) and in the transaction that i make i put 0.01
BTC in front of your bitcoin address to pay you that much and i am left with 0.025
BTC.
now comes the fee part. i can send this 0.025
BTC back to my original address or a new (change) address and how much i send there determines the fee. if can sent the whole thing and fee will be 0 or send 0.02 and fee will be 0.005
in other words there is no variable in a transaction saying fee is XX. instead it is calculated automatically based on leftover.
the other thing i can do is i can select all the outputs (all 6 of them total of 0.16023) and make a big transaction and send you your 0.01
BTC and be left with 0.15023
BTC which i can send 0.15 to a new address and fee will be 0.00023
BTCnow what i mean by this is that it doesn't matter where those transactions are coming from (0a927f86e75.... and 13a4c5470a.... can belong to different addresses) in the end you are left with that total anyways and you can send the leftover to address A or address B or a new address C
makes sense?