It depends on your proficiency, long term plans, and budget. SLI can cause more issues and will have more incompatibilities spurred on which will cause you to have to troubleshoot and mess with things. Some people like this, some hate it. If you don't plan on upgrading the rest of your components for a long time, SLI might not be the worst option, but you'll get better performance from one 970 than the two 760s. If you plan on keeping your current hardware, I'd grab a 970 and possibly SLI that down the road. SLI might be a bit more cost efficient upfront, but you will be pulling more power from the wall, you will probably need to reconfigure your airflow if it's not on point which could lead to additional costs in fans, pwm hubs, cables, etc. There's obviously pros and cons to both, but when I make decisions like this, I try to think big picture and what will be best overall.
One last thing to note, if you had power issues previously, which raidmax are notorious for, there is a possibility your current 760 will not function as long as you hope and your SLI dream could go up in smoke. PSUs are never an area to skimp, so you might want to invest in a new graphics option just to be more safe in the future and start anew.