Allow me to add to Don's great advice. I'm not one to buy lots of large batt mods, and not just because that would be expensive. I want convenience to carry and use my PV easily when out and about. I am also not a big fan of really high wattage. Many find the 10-12W range. such as a 5V PV with a 2.5 ohm 510 atty, their "sweet spot". Watts=Volts*Volts/ohms. I prefer around 5- 6 W, which is what I get with a true 3.7V mod using the 14500 batts, and a normal 510 atty.
Right now I am really enjoying the Bartleby:
Bartleby
It has an on/off master switch, so you can put it in your pocket without danger of setting it off. It is small, well-built, fits nicely in the hand, has a big activation button, and can be fitted for 510 or 801 attys. It packs considerable more oomph than the regular or mega 510 batts, but not so much that you fry attys, and frying attys is one of the main problems with the HV mods. It also recharges from USB, and can be vaped while charging, so is a PT. And it charges
fast from USB. The Bart is an example of a "box mod", and runs about $50, so is less expensive than most of the other mods. Jay makes a really great product and stands by it.
It won't win the fashion shows like some of the other mods, but it does its job
very well. I think regular 510 attys operate far better on a batt like the 14500 (900 mAh), or with a higher capacity 3.7V batt, since the wattage will be higher than on a regular batt, and the atty tends to stay unclogged. You can rev up the wattage on it by getting a low resistance atty (~1.8 ohm), or dial the wattage down a bit with a DSE510 atty (3.0 ohms). And from personal experience, I know that in general my attys last longer on this battery than the regular or megas, mainly due to the self-cleaning the slightly higher wattage promotes. My attys are lasting about 4 months now, rather than 4 weeks, and some that were chronically clogged before on the reg or mega batts are now opening up.
For me money is an issue, and I like a mod that can deliver the power I like, and also help preserve my precious attys...and carts too, since HV mods can tend to melt them. And 14500 batts are cheap.
Just my $0.02. As Don said, though, figure out what you want, including your daily using habits (work, home, on the go, etc), and read as much as you can from other posts on ECF. I researched mine for several weeks before making the jump, and I'm glad I spent the time.
Good luck, and hope you find a good one that works well for you!