Hushmail is definitely cheaper since Hushmail is free and SecExMail is $4.95,
As far as security, either option has its risks. Hushmail stores the mail on their servers and SecExMail saves them on your computer. If you are really in trouble with the law, it shouldn't be hard for the government to subpoena Hushmail Communications just as easily as they can get a search warrant to search your home. If they subpeona Hushmail Communications, they don't have any obligation to notify you that they've got copies of your emails from Hushmail Communications directly and how many times they've done it. With a search warrant, at least you know they're investigating you when they come to your door.
This makes the point moot when comparing Hushmail to SecExMail in securing the location that your emails are stored. The only benefit of Hushmail, as mentioned earlier, is the convenience of access from any web browser vs. access at computers with SecExMail installed.
If you feel more comfortable keeping your mail on your computer and you're already using Outlook XP or 2003, you can encrypt your Outlook Personal Storage Files or PST's with a password. You just have to turn on the option in your mail control panel. You can additionally re-encrypt it at the file-level using Windows XP Pro's EFS (EFS is disabled in the home edition). This way, not only do you get protection during transit, you're making it a little harder for the fed's to access your mail if they do get physical access to your hard drive rather than Hush Communications just handing over your mail to the government.
If you're lucky and get warned that the fed's are coming, you may even get enough time to do a data wipe to Department of Defense recommendations and if you're really lucky, Gutmann recommendations, before the fed's gets their hands on your hard drive.
You can avoid paying for a secure relay agent like SecExMail and use solutions that are freely available although they require a little more tech savyness. Those include PGP 8.0.3 freeware and GNU Privacy Guard.
Another option is to try the encryption that is built into Outlook, S/MIME. Although most companies like VeriSign charge, there are several Certificate Authorities that offer X.509 certificates for free. The advantage of using a X.509 certificate for S/MIME encryption is that you can use the same X.509 certificate to encrypt your AOL instant messages on AIM versions 5.2 and above. AIM does not support PGP encryption in any version.
Hushmail uses OpenPGP. Because PGP and S/MIME are not compatible, the choice on which encryption method to use will be dictated by the encryption method that your contacts use since encryption requires that both parties have the same type of encryption key. SecExMail uses neither standard. It uses double encryption via 64 bit ISAAC and 256 bit Twofish encryption. Thus, it is not compatible with PGP or S/MIME encryption.