Is there any major difference between Truffle's Ganache and TestRPC? Do they run irrespective of each other as test blockchains, are there any side effects running both at same time? Is the only improvement that Ganache has a more handy ui?
Ganache is TestRPC, it is like a rebranding. The github repo for ethereumjs-testrpc now redirects to ganache-cli.
As of today 2017-12-20 if you install through npm, both
npm install ethereumjs-testrpc and
npm install ganache-cli will install the same application.
You can launch both at the same time, but you have to select a different port for the second instance (only one instance can listen from port 8545).