I'm trading with both. Oanda works better with small positions, on IG you really want to have min 10k per position due to (large) commission.
On Oanda's financing charges: https://www.oanda.com/us-en/trading/financing-fees/ or (if you're not US bound and need to know about indices/commodities?) https://www.oanda.com/sg-en/trading/financing-costs/
IG has a lot of other stuff than forex and indices to trade though, so if you need to mix in e.g. equities that would obviously be preferable.
As for these "brokers" "trading against you". You can compare their prices vs another price source. If they do stand on the other side of your trade or not, it otherwise doesn't matter if you get a market based price (beyond counter party risk). What does matter is the spread that's going to be larger than on market, and that is also manipulatable. I haven't excessively seen that from these two though, it's clear Oanda is keeping me in the trade as long as possible.
For the record, I also have IB but in my country IG is more convenient (also than Oanda) due to better interfacing with the Swedish tax system.
As for shorting US dollar: Make sure you actually understand what you're doing and not just superficially do so. You're not going to make money based off the latest clickbait headlines on e.g. Zerohedge.
Thanks Snuskpelle! Same question for you as I asked JSOP above. Also, you said "it's clear Oanda is keeping me in the trade as long as possible" - that sounds kind of scary. What are they doing to to try and keep you in the trade? I don't like the sounds of that one bit.
Thanks!