Which do you think is easier to rank on page 1 for? High competitive keyword phrases that may require thousands of quality inbound links (maybe more) to even make it to the top 50? or low competitive keyword phrases that you might be able to rank on page 1 for with no links or only a few?
I would rather rank on page one for a low competitive keyword phrase that only bring me 50 visitors per month than to rank 90th for a highly competitive keyword phrase because I will get ZERO visitors from that...
50 visitors > 0 visitors...
Over time as you start to rank for lots and lots of long tail, low competitive keyword phrases... you'll have a better chance of eventually ranking for your competitive, head terms.
Unless you have the marketing skills to get lots of backlinks from relevant, authoritatitive sites with your head terms used in the link text, you are typically better off going after the long tail.
PS: Don't confuse search query volume with competitiveness. They are two entirely different things...
If you use Google's External Adword Keyword Tool, you can check the EXACT match type search query volumes to get a relative feel for how many people are searching for a phrase compared to other phrases.
But you need to look at how many sites have that EXACT keyword phrase in their <title> and URL to get a real feel for how many sites are actually "trying" to rank for that phrase.
I search Google lots of ways including searches like:
"my keyword phrase"
"my keyword phrase" intitle:"my keyword phrase"
"my keyword phrase" inurl:"my keyword phrase"
"my keyword phrase" intitle:"my keyword phrase" inurl:"my keyword phrase"
to check the competitiveness of "my keyword phrase"... I look at all of the results to determine the overall competitiveness of the phrase. You can use allintitle and allinurl as well.