But can't that big competitor with lots of money do what you're doing?

Yes they can copy you and your dilemma is justified by looking at your position. However think about this

1. Why will they copy me. They can only think of copying if you grow in the differentiated version. If you grow you have enough power to compete with them...
2. Copycats are always there but you have a first mover advantage. Even if you are not radically different You 'Position' yourself well in the minds of the market

Lastly I feel a veteran investor will not ask such silly question but lets say he still does that counter question him this --

Gmail is the best free email available. Cant a dying Yahoo, Hotmail or AOL copy them. When Gmail came up with a 4 GB limit others copied this with even bigger disk sizes, some went to the extent of unlimited mailboxes. Did the customer jump back to them ?

It seems like the best way you can validate your idea is to get a number of customers signup and pay for your service in some form. If you show traction investors will pay attention.

As well as the other ideas:

"Sure they could do it. But by the time they have worked out that it's worth doing we'll have a real market presence, and they'll be behind the curve. Then we'll sell the company to them."

It isn't about copying the idea of the features, it is about whether they can actually do it too. Perhaps the big company has positioned its products so it can't do what you are doing. They may have segmented the market and are ignoring the low end because they can't effectively reach them and sell to them (and make a profit). Or their products are expensive and lower priced product would undermine their price list/value story. Perhaps the decision makers they target are not the same decision makers you will target. The reasons are more likely to be sales/marketing decisions and not technology decisions.

The best answer to investors is that you will be targeting the prospects that are not customers of the big company, and a possible exit strategy is being acquired by the big company to get those customers.

Or, depending on your market research, another answer is 'this market is so big they won't notice me for a while'

Some ideas just aren't big enough for Goliath. Goliath has huge costs (administration, layers of beuracracy, big advertising and sales budgets, management, etc). Unless they can make several hundred million dollars per year on this idea, it simply might not be economically feasible for them to go after your niche/market/twist.

You tell them exactly what it is you can do that Goliath either can't or is very unlikely to want to do.

For example, if you were competing against google, "excellent customer service" would be one such point. Google's customer service is a black hole of despair; as a small company, it would be quite easy to do better than that, and you don't have to be particularly worried that they're going to go and copy you, either.

Every big competition has weak spots, things they just don't want to bother with, usually because they don't scale well (customer support, creating a personal connection with your customers, having a quirky and unique web presence...)

You say you're "uniquely differentiated". Look at the list of things that make you different, cross out the ones a big company could easily and profitably copy just by throwing some money at the problem, and there you have your answer.