There are typically 3 ways to do this.
1. Install a transfer switch between meter and existing panel. Or the behind the meter one you mentioned. This can be a manual or automatic switch. This is probably the most expensive(other than the behind the meter one you mentioned), usually due to the cost of the switch. It can also add up depending on how much has to be relocated to accomidate the switch installation. the meter also needs to be removed/replaced by utility company.
2. Replace the existing panel with one designed to be backfed. These panels have an interlock, usually a sliding metal plate, between the main input breaker and one of the 2 pole breakers. The generator is connected to the 2 pole and the interlock plate only allows one of the breakers(main or generator) to be on at a time. This prevents backfeeding generator power to the grid. This is probably second in line as far as price. The panel is probably cheaper, but there is a lot of labor involved. This also requires the meter to be removed to isolate the panel.
3. Sub panel for generator fed devices. There are 2 ways of doing this:
A. Done similar to #1 above, with the transfer switch between main and sub panel. You must relocate all the services that are to be fed with the generator to the sub panel so there is a bit of labor involved.
B. Addon generator specific sub panel. This type installs beside the main panel and is spliced into the main panel at the breakers that are being configured for generator feed. With main breaker shut off so panel busses are deenergized, you remove a wire from a particular breaker. You put a wire to the sub panel in the breaker. You connect the original wire that goes to the household outlet or device to a second wire from the sub panel. You do this for each circuit being configured for generator power as well as tying into round and neutral wiring. This type subpanel has a 2 way switch for each circuit. When bringing up the generator, you switch each circuit from commercial(fed from it's original mainpanel breaker) to gen in the sub panel. This is probably the least expensive option and doable by someone who understands how to work in a panel safely and can follow directions. These are typically used in smaller generator aplications where only partial/vital circuits are powered.
Wanting whole house transfer, sounds like you want 1 or 2. Most utilities I Have experience with want a liscensced electrician doing anything up stream of the panels main breaker, and you will need the meter pulled and re-installed/resealed after it is inspected. If you can find a good source for an approved switch, #1 probably involves the least labor IMO. #2 is probably the most cost effective for a new home build/panel installation.
I am not sure how an inspector would look on the modifying of an existing panel to option #2 by the addition of a sliding interlock plate to allow safe backfeeding of the panel. The new panels of this type that I have looked at do not differ in any other way that I could see from a standard residential panel other than the sliding/rotating interlocking plate that is riveted/permanently affixed to the panels breaker cover plate. Providing the panel can support a breaker large enough to provide for the infeed power from the generator (15KVA(KW) at 240VAC is 62.5A), this would be a VERY inexpensive option...