Ok,
You said 'A second question' ? i can not see a question beside those '1. What is the difference between exchange buy/sell and margin buy/sell - and what exchange, would you recommend for trading?' ? or i missed something in here ?
or you're asking what is the best wallet for you to stock your BCC/BCH without downloading blockchain ?

i have no idea about BCC/BCH wallet,but i only can warn you about this.
a lot of thing happened in BCC/BCH wallet such as 'ELECTRON',you can found it anywhere many people complained about it.
so from here i could find a clue that BCC/BCH have a problle in thier network or system (or i might be wrong).
and of course,other than that i warn you to invest your money into BCC/BCH right now,because you can not deposit your money right now.
just wait a bit longer because when deposit/withdrawal can be used a 'HUGE DUMP' will happen.
and just use a wallet from exchanger that supporting BCC/BCH.

About the difference,
Margin = Borrowing / Leverage and Position
Leverage = Loan
Margin = Position ( Selling or Buying ) or ( Holding or Shorting )
Margin Sell = Leverage,but you get a profit when the prices is down
Margin Buy = Leverage,but you get a profit when the prices is up

in brief :
Leverage,or we called it Loan that mean you can borrow bitcoin/altcoin in their exchanger based on their term,from what i can see many exchanger can do 1-2,5x Leverage ( that mean 1-2,5x from your initial amount[capital] ) some of them can do 1-100x Leverage ( that mean 1-100x from your initial amount[capital]).
Margin/Position in reality actually same like Leverage,we're borrowing bitcoin/altcoin,
Long that mean holding and Short that mean Selling.
Long/Holding right now bitcoin price around 2000$ and you have a money on exchanger around 4000$ that mean 2BTC,you use Margin Long 2,5 and you can use 10000$/5BTC and Bitcoin price up to 2100$.
you used 2,5x Margin Long that mean you have 5BTC and price up 100$ that mean you get profit 100$x5=500$
Margin Sell/Shorting same like Long but you get profit if the price goes down.

BitMex might be ? because my favorite exchanger aka 'POLONIEX' doing a shady activity these days,so i would not recommend it