If you are thinking about an eroll -- you're probably thinking about portability and the small form factor/reminiscent form factor. I would instead suggest an itaste vv v3 or itaste mvp v2 as a great place to start. I'm a recent convert myself, and started on an ego battery (which is essentially an upgraded 510), but quickly moved on after that, and in retrospect, wished I could get my money back from the ego batts. I really like the itaste vv v3, as it is not much bigger than a 510, adds ego threading, and has vv/vw and a passthrough connector, which also happens to be micro usb -- the same as most non-apple phone chargers. Passthrough (or semi-passthrough as I guess this is) just means you can use it while it's charging. The problem you will run into with a 510 is having to charge it during the day, or have multiple batteries -- charging a 510 cannot be accomplished while it's being used, and requires a proprietary charging cable, and the included ones are usually painfully short.
MVP 2.0 is also a great choice, as although it is rather large compared to what you've been considering, it also has a comparably massive battery...2600mah will keep you vaping for 24 hours for sure, and maybe 48+ depending on your vaping habits. Also adds the functionality to be able to charge other devices off your battery, which, if useful to you, takes away some of the sting of carrying a small brick in your pocket. It's not that big...about the size of a pack of cigarettes, but about an inch narrower -- just pretty heavy.
Just better performing devices which give you some room to grow. Pair the itaste vv with a protank 2 mini, and you're off to a really good start. itaste vv v3 can be had on ebay for just over 30 bucks shipped, and the mvp about $46, which includes tanks, and everything you need to get started (except juice of course).
With the e-roll or 510, you're going to hit the ceiling for usefulness and flexibility very quickly...with the devices I suggested, you can get a pretty darn good vaping experience for awhile before you might get the urge to start splurging on fancier stuff. I really believe the mvp 2.0 is a great place to start, and an unbeatable value.