A Garmin, a fancy Garmin, a s/h power system or what?

hi, and thanks in advance for your patience

Im a keen middling sort of rider in many ways
I ride with a bunch of regulars and although not the fastest Im certainly not the slowest

I have just returned from watching the Tour and comfortably rode to the top of Tourmalet via another Col to get there

Whilst abroad I have been talked into next years Etape
To be honest I was kicking myself for missing this years

In the meantime I have 500 pounds to spend on making myself faster
Mainly to get closer to the mate I rode with in the Pyrenees who is on an althoether different level

I have a nice bike with an Ultegra groupset Im happy with and nice wheels Im not even thinking of upgrading

The money will be spent on something else, thoughts as follows:

Garmin 500 - although apart from data logging Im not sure what else this serves as I have an accurate conventional HRM/bike computer

Garmin 705 - this interests me from a training point of view, but having looked at the manuals online exactly how useful is it and what do you do with it in reality?

Power stuff - this is what my fit mate is raving about. 500 nicker is pushing it I admit but if Im patient and syphon off some of the housekeeping is a possible

Winter bike - the clever moneys here, but I do not ride thro the dark months as I hate the roads this time of the year, however I do regularly use my MTB with lights, Im not sure a winter bike would actually get me out that much

Personal trainer - again an outside option, with a young family my riding opportunites are precious and any more time on the bike tough to justify

So comments welcome
I admit its a series of questions with no answer and Im fortunate to have good biking DNA but I do have some spare cash and spending it on bikes would make me happier

For me its the 705 vs waiting to buy some SRM cranks at a good price

Dont be shy - Im a big boy LOL