Amy,
HELOCs vary in terms but will typically have a "draw" period where you can use it like a credit card and only be on the hook for interest payments. This allows you to tap into the equity of your home with a lot more flexibility. If you need it, it's there. If you don't, it doesn't cost you anything to just sit on it (assuming you haven't pulled any money off of it). If you have the opportunity to pay it down for some reason (i.e. inheritance, flip, etc.) you can easily access it again whenever you want. That is why it is ideal for strategies that allow you to cash out, either through flipping or refinancing. You will be able to move faster than people who have to secure financing because you do not have to ask to borrow money on your HELOC. Essentially, you will be doing cash deals. If you don't cycle it though, you will eventually tap out the HELOC and be looking for capital again.
During the interest only period, the payments on a HELOC are usually very reasonable. I have several HELOCs at prime (just above 3%) from the glory days of lending before the crash and one at 5.75% at 90% LTV from after the crash. Always remember, though, that after the "draw" period, there will be a repayment period where any outstanding balance will be amortized for some period of time. For example, you may have a 10yr draw period that will switch to a 20yr repayment period. That means your payment on the balance will jump considerably when it switches. That is kind of a worse case scenario. I have one HELOC like that. Others that I have are 20yr draws with a 20 or 30 year payback and one that they just keep renewing the draw period as long as I keep making the payments. Shop around for the best rate and terms that work for you and your strategy.
If you refinance your house you will have a pile of money that you will need to employ for it to be profitable. Even if you never do a deal you will be paying for that cash and your mortgage payment will be much higher too. Also, if you decide to pay down your mortgage with a windfall of some sort as mentioned above, not only will your payment not go down like it would with a HELOC, but you will not be able to get that money back without refinancing again or selling your house, unlike the HELOC. If it is not obvious, I would definitely recommend the HELOC over refinancing.
Always remember too that these loans are on your personal residence. So if you blow the money and can't make the payments you will lose your house. I don't say that to discourage you or scare you but to ensure you understand the potential consequences and adopt a strategy accordingly. I would certainly be more conservative with this money than I would other sources of financing. Also, if you are responsible with your money, I would highly encourage you to get the HELOC regardless. Just having access to that kind of money is very powerful, assuming you only use your powers for good.
As far as wholesaling is concerned I would not dissuade you from giving it a go if you think you have what it takes. Personally, I know that is not for me. Those are the guys that find the best deals, though, and I completely agree with the person who urged you to do it and cherry pick the best deals for yourself. Would that I could do that. It does take an investment of time and money though, and it is not nearly as easy as many gurus make it out to be. Still, many people, and many people on BP, have been very successful doing it.
Ed