I would def get the lavatube. The eGo VV really isnt a true VV device. I guess technically it is, but your limited to only 3 voltages, 3.2V, 3.7V, and 4.2V. And its really actually more like 2.9-3.0V, 3.4-3.5V, and about 3.9V. You will notice a difference at the highest setting from a regular eGo since they are only 3.3-3.4V, but it really wouldnt be a huge difference from a 3.7V PV like the kGo or E-Power. Heck, the kGo would even prob give you a higher voltage vape since it comes off the charger at 4.2V.
The lavatube is a true VV device and you can dial in the voltage in .1 increments from 3V-6V. It will be a BIG difference from the eGo VV. Also the lavatube uses non-proprietary batteries which are a lot better. They are cheaper and last a lot longer before they need to be replaced. And the non-prop batteries will last a lot longer before they need to be recharged. You would also have the ability to carry some spare batteries if needed for the lavatube. If a battery dies, you can pop in a fresh one and are good to go. With the eGo, youll have to wait for the battery to recharge, or youd have to purchase another one to be able to carry a spare. You think you have long battery life now, just wait till your using something that uses a 18650 battery. I know you like the eGo, but I think you really would be a LOT happier with a true VV tube mod.
Also, have you ever used any other attys or cartos other than the eGo-T attys? Cause there are quite a few options that are a LOT better than the eGo attys/tanks. Ive used the eGo tanks before and I personally think they are awful. They were a good concept on paper, but it really ends there. I find that the tanks dull flavors and TH. Plus their finicky and leak constantly.
I know everyones different and if your happy with them then thats great. But I feel like if you use only them on a true VV device, your really kinda limiting its potential. It would be like putting a 35-mph governor on a Ferrari.

I know you like them and are comfortable with them, but I think if you open up to some other options, youll find there are things you can use that are a LOT better. You could get a nice carto tank that blows away the eGo stuff. Also, since the lavatube voltage can be changed in .1 increments, you could essentially buy pretty much any ohm carto to put in it, and dial it in to find your perfect vape. And thats the reason that people love VV devices so much. It allows you to truely find your perfect vape. On the eGo VV, you could use LR attys/cartos because frankly, all 3 voltages are low. With a real VV PV, using LRs is pretty much pointless because it would limit you to only the lower voltages. Even at 4.5-5.0V, youd probably pop the coil on a LR attys/carto. I would use a 3.0ohm carto or higher, because then theres a MUCH larger range of voltages to use and you can really dial in your preference. With the eGo VV, your kind of limited bc it only goes up to 4.2V, and its really only about 4.0V, which really isnt that high at all. Most people that use VV find their sweet spot at around 5-6V, and with the eGo VV, youll never even get to experience that. With the eGo VV, the different voltages are basically still in the same range. You will taste the difference in some juices with the eGo VV, but it will mostly just be minor differences. With a true VV, the much larger range of voltages really makes a difference. I think youd be quite surprised how different some juices taste with a larger range of voltages. A juice at around 3.7V can taste quite different at 6V+. Some complex juices that involve many different flavors have undertones of different flavors that go unnoticed in the 3V range, but really come alive at around 6V or more.
Personally, if I was making the decision between the two, Id pick the lavatube bc I feel the eGo VV is not in even close to the same league as a true VV tube mod, but thats up to you.
Personally, I dont really consider the eGo VV to even be a VV device.
Heck, if you really dont want to get the lavatube, I would at least get something like a Maxi Roughstack then. Its a 3.7V tube mod and isnt considered a VV device at all, but at least with that you could stack different batteries and vape at 6V and 7.4V. I consider even that to be more of a VV device than the eGo VV, even though its not, but hey, thats just me.
Goodluck!